GRE is NOT an english proficiency test like ielts / toefl: its a graduate admission measurement test (measures verbal reasoning, quantitative reasoning, critical thinking, and analytical writing skills), based on which scholarship is determined, especially in U.S. universities. Check the official site for general overview.

Before GRE:

But GRE is not the first step to apply in foreign universities: the first step is the English proficiency tests like IELTS or TOEFL. As soon as you are done with your bachelor, before the result comes out sit for either of these tests and get desired marks. Consult with the unviversity web page which test is more suitable for them. Some times there are other issues - even though you have good score in TOEFL, and even if you get admission, to get visa in countries like Australia, you will have to take IELTS.

So, check first which test is more suitable for you. As you write your exams in english, this should not be a big problem anyway (hence no need to take coaching for this, according to my opinion) - but do practice the spoken english for a while if you are not used to it (usually we are not, being in a non-english speaking country).  All in all - should not take more than 2/4 weeks (assuming you are from bangla medium, which varies a lot depending on your exposure in english). Take some mock tests in saifur's (based on my experience - they have some quality with in less price). But if you are talking about real quality - go for IDP mock tests - although they are pricy!

After taking those English proficiency tests and achieving desired marks, you should go gor GRE if your target university/department requires it.

On a personal note, i found watching BBC all day long helps a lot (although the contents might be so boring)! I used to watch it so much that even now, sometimes i find myself watching BBC in my leisure times.

Getting Started:

To get started, do something yourself and i'd definitely suggest you to solve all math books before class 8 and buy a latest edition oxford dictionary from nilkhet. That will get you going - at lest its a start. do this by yourself at first without doing anything else.

My Recommendation about Preparation:

For gre, i'd advice you to take 3-4 months and not more than that, and during those months you have to study hard as any other exam and cannot do anything else (like job / isrt exam / class and stuffs like that). if you plan to do it with something else, then it takes so long time that you'd probably forget the techniques you had learnt in the initial stage. That is my suggestion.

For gre, i think it is best to be with some group (like you should continue with a coaching centre, not for the lecture - they are not much effective anyway, but for going along with a group who will be will you all/most of the way). But it costs a lot, so think about it. Saifurs / gateway / other coaching centres are there - get information about which one is running now. Go with one which has most students and get yourself admitted there. As soon as you finish ur 1st 3 months - get registered with ets for final exam.
 

My Favourite Books:

Before going to any coaching centre, buy some books from "khan book centre" of nilkhet (its beside modern book centre). get kaplan's (i like this series more - because they have lots of book in specific topic) and barron's (this series has some unified books -cheap and popular) gre book and try to read by yourself for somedays before you decide to go to any coaching centre. Its a long process and there is no limit how far you can read. Therefore i suggest to be with a group.

For GRE words, please try to buy the "Word Power Made Easy" by Norman Lewis from nilkhet right now (i bought it with 90 taka in 2006) and start memorizing at least 3 words each day: this should not hamper your studies much. When you try to do gre within 1 month after your masters/honors, its nearly impossible to memorize all these words within such a short time (8,000 gre words are at least necessary: and toefl/ielts vocabulary is some thing that you need to use everyday life, but gre vocabulary is something that you never use, but that exists in dictionary). But if you start from now, it will not be a burden, rather it will be fun. Involve your friends if possible and do it together. My suggestion is to start with Barrons / Kaaplan's high frequency words, and later expand.

Some websites might come handy - such as this or this. Search in esnips or google it! One great source is google with the option "define". Type define:snip in google to see what happens (another reason why i love google)! Such words are also useful in several other places (you will find out when its time).

GMAT is closely related, but not the same. Still some of the gmat books might be useful. Check the content of those books at least to find out.
 

Summary:
 

To recapitulate:

1. do all maths before class 8
2. buy a dictionary - the oxford latest on advanced learner's (but don't start memorizing right away), you will need to start with High-Frequency GRE Words at first
3. buy kaplan and barons book and try reading by yourself
4. get admitted in a coaching centre, if you decide to do that
5. get some friends who also doing gre, may be from coaching and practice with them
6. after you get to know the syllabus, make a schedule of how you continue
7. sit for gre exam within 3/4 month, not more than that, obviously if you think you r prepared enough - because these are VERY expensive exams and you don't need to waste so much amount of money if you are not confident enough.

Check other comments as well before you decide to go for this! This is a long term commitment - so you should know where you are getting in before you actually do it.

Good luck with your preparation. 

[This is an extended version of something that I wrote before somewhere else based on some email correspondence I had with a few interested students regarding GRE]