Answering the first question here: 

What is nirvana, how to achieve it?

To answer your question let me draw an analogy. What is required to become a professor? According to Wikipedia, "A professor is a highly accomplished and recognized academic, and the title is in most cases awarded only after decades of scholarly work to senior academics". Being a professor is not a concrete state, it is a label they put on years of hard work, culminating in professional realization.

Similarly, Nirvana is not a concrete state, it is a common name for the condition of a highly accomplished spiritual practitioner. This condition is not something you can contrive knowing the requirements, it is a result of years of personal transformation.

Nirvana can be characterized as condition of having transcended suffering & death. The requirement for this is said to be a complete cessation of Attraction (obsessing over something as desirable), Rejection (obsessing over something as undesirable), and Ignorance (mistaken understanding of how everything works). One of the key components of Ignorance is a deeply lying conviction in existence of substantial self and a tangle of reflexes that grow around that. Much of Mahayana practice is targeted at destroying this belief, not just at conceptual level, but at the level of basic day-to-day instincts.

If we could simplify the requirement for becoming a professor to "complete cessation of ignorance in the field of study one specializes in", we could perhaps make a stretch and define Nirvana as "complete cessation of ego complex, along with negative experiences and behavioral tendencies it generates". As any simplification, this is not entirely accurate: just as becoming a professor requires tons of practical skill and experience outside of the primary field of study, attaining Nirvana requires cessation of all mental and emotional obscurations, a tendency to get stuck on anything, not just on self.

Now, for the purposes of this discussion I'm equating Nirvana with Enlightenment, which depending on a school may or may not be the right thing to do. Specifically in Mahayana, Nirvana is considered a conceptual projection (a shadow of ego so to speak) that has to be transcended as part of awakening to Self-Existing Buddha Nature (=Enlightenment).

In Buddhism you need to follow the Noble Eight Fold path to attain nirvana. It is further divided into three parts for simplicity - Sila (morality), Samadhi (mental culture) and Panna ( wisdom).

The noble eight fold path is as below : 

Sila (Morality): 1) right speech 2) right action 3) right livelihood 

Samadhi (mental culture): 4) right awareness 5) right concentration 6) right efforts 

Panna (wisdom) : 7) right resolve 8) right view

In whichever teaching the components of Noble Eight Fold path is found there you will find the source of Nirvana. So you can follow any sect you prefer. But the right view is what differs in Buddhism which takes you in the right direction. Understanding Buddhism's (right) view of rebirth, karma, and three realities of life (impermanence [the state or fact of lasting for only a limited period of time.], no soul, and suffering) and dependent origination can help understand Nirvana better.

Five Hinderances

Ponder these.

1. lustful desires (kāmacchanda)
2. ill-will, hatred, or anger (vyāpāda)
3. torpor, lethargy, laziness and languor (thīna-middha)
4. restlessness and worry (uddhacca-kukkucca)
5. sceptical doubts (vicikicchā)

Seven Factors of Enlightenment

Meditate on these.

1. Mindfulness (sati) i.e. to be aware and mindful in all activities and movements both physical and mental.
2. Investigation and research into the various problems of doctrine (dhamma-vicaya). Included here are all our religious, ethical, and philosophical studies, reading, researches, discussions, conversations, even attending lectures relating to such doctrinal subjects.
3. Energy (viriya), to work with determination till the end.
4. Joy (pīti), the quality quite contrary to the pessimistic, gloomy, or melancholic attitude of mind.
5. Relaxation (passaddhi) of both body and mind. One should not be stiff physically or mentally.
6. Concentration (samādhi).
7. Equanimity (upekkhā) i.e. to be able to face life in all its vicissitudes with calm of mind, tranquillity, without disturbance.

One day Malunkyaputta got up from his afternoon meditation,went to the Buddha, saluted him, sat on one side and said:'Sir, when I was all alone meditating, this thought occurred to me: 

There are these problems unexplained, put aside and rejected by the Blessed One. Namely, (i) is the universe eternal or (2)is it not eternal, (3) is the universe finite or (4) is it infinite, (5)is soul the same as body or (6) is soul one thing and body another thing, (7) does the Tathagata exist after death, or (8) does he not exist after death, or (9) does he both (at the same time) exist and not exist after death, or (10) does he both (at the same time) not exist and not not-exist. 

These problems the Blessed One does not explain to me. This (attitude) does not please me, I do not appreciate it. I will go to the Blessed One and ask him about this matter. If the Blessed One explains them to me, then I will continue to follow the holy life under him. If he does not explain them, I will leave the Order and go away. If the Blessed One knows that the universe is eternal, let him explain it to me so. If the BlessedOne knows that the universe is not eternal, let him say so. If the Blessed One does not know whether the universe is eternal or not, etc., then for a person who does not know, it is straightforward to say "I do not know, I do not see".'

The Buddha's reply to Malunkyaputta should do good to many millions in the world today who are wasting valuable time on such metaphysical questions and unnecessarily disturbing their peace of mind:

'Did I ever tell you, Malunkyaputta, "Come, Malunkyaputta, lead the holy life under me, I will explain these questions to you ?" ''No, Sir.''

Then, Malunkyaputta, even you, did you tell me: "Sir, I will lead the holy life under the Blessed One and the Blessed One will explain these questions to me " ?''No, Sir.''

Even now, Malunkyaputta, I do not tell you: "Come and lead the holy life under me, I will explain these questions to you". 

And you do not tell me either: "Sir, I will lead the holy life under the Blessed One, and he will explain these questions to me".Under these circumstances, you foolish one, who refuses whom? 

'Malunkyaputta, if anyone says: "I will not lead the holy life under the Blessed One until he explains these questions," he may die with these questions unanswered by the Tathagata. SupposeMalunkyaputta, a man is wounded by a poisoned arrow, and his friends and relatives bring him to a surgeon. Suppose the man should then say: "I will not let this arrow be taken out until I know ho shot me; whether he is a Ksatriya (of the warrior caste) or a brahmana (of the priestly caste) or a Vaisya (of the trading and agricultural caste) or a Sudra (of the low caste); what his name and family may be; whether he is tall, short, or of medium stature; whether his complexion is black, brown, or golden; from which village, town or city he comes. I will not let this arrow be taken out until I know the kind of bow with which I was shot; the kind of bowstring used; the type of arrow; what sort of feather was used on the arrow and with what kind of material the point of the arrow was made." 

Malunkyaputta, that man would die without knowing any of these things. Even so, Malunkyaputta, if anyone says: "I will not follow the holy life under the Blessed One until he answers these questions such as whether the universe is eternal or not, etc.," he would die with these questions unanswered by the Tathagata.'Then the Buddha explains to Malunkyaputta that the holy life does not depend on these views. Whatever opinion one may have about these problems, there is birth, old age, decay, death, sorrow, lamentation, pain, grief, distress, "the Cessation of which (i.e.Nirvana) I declare in this very life."

'Therefore, Malunkyaputta, bear in mind what I have explained as explained, and what I have not explained as unexplained. What are the things that I have not explained? Whether the universe is eternal or not, etc., (those 10 opinions) I have not explained.Why, Malunkyaputta, have I not explained them? Because it is not useful, it is not fundamentally connected with the spiritual holy life, is not conducive to aversion, detachment, cessation, tranquillity, deep penetration, full realization, Nirvana. That is why I have not told you about them.

'Then, what, Malunkyaputta, have I explained? I have explained dukkha, the arising oi dukkha, the cessation of dukkha, and the way leading to the cessation of dukkha1 Why, Malunkyaputta, have I explained them? Because it is useful, is fundamentally connected with the spiritual holy life, is conducive to aversion, detachment, cessation, tranquillity, deep penetration, full realization, Nirvana. Therefore I have explained them.'