i) What to write:
Write the celebrity a letter explaining what you are doing, why you are doing it etc. 3 main points most people include:
1. A bit about yourself
2. A bit about what you are collecting for, where the money will go etc
3. Thank them for their time.
If it is a celebrity that you particularly admire, or you feel it would be better to include something about a recent movie/concert/album/book etc, then add that in too
Try not to make your letters too long, because many celebrities are very busy, so may not have time to read your letter. Try to make sure it is no more than a page. Short, Sweet and To The Point
ii) Who Replies
That really depends. Many celebrities reply, and I am talking literaly thousands. The best way to do this is probably to make up a list of celebrities that you admire or you want to write to, then searching for them in the database. Most well-known celebrities have many feedbacks reported, and a lot of information about whether or not they reply can be found by looking at other peoples sucesses.
Remember that even if the celebrity has a lot of feedback, does not mean that they are good signers. Watch out for Preprints, Ai6topens, Secretarial, Forged or Stamped autographs. It is best to look at a few successes and compare the autographs to see if they are genuine. Also, read other peoples comments as they are often vert helpful
More on how to tell if your autograph is genuine here:
http://www.fanmail.biz/mboard/viewtopic.php?t=9991
iii) What Postage To Use
This is where I kind of get helpless

You should always try to include a SASE (Seld Addressed Stamped Envolope) when writing to celebrities who live in the same country as yourself. Just make sure that you use enough stamps. As for celebrities living outside your country, I think you need to use IRC's (International Reply Coupons), but you would probably be better asking someone else about this.
For more information on IRC's go here:
http://www.fanmail.biz/mboard/viewtopic.php?t=14171
For more information about SASE's, SAE's and SSAE's, go here:
http://www.fanmail.biz/mboard/viewtopic.php?t=14169
For help with postage costs go here:
http://www.fanmail.biz/mboard/viewtopic.php?t=14167
Hope I have helped

I am not an expert, or anything close so you may want to check with someone who knows a bit more ;)