Answer:
Congratulations on making this type of commitment, it takes an incredible amount of dedication and discipline to compete in a show!
First of all make sure that you're ready, meaning that you have built up some muscle mass, that is what bodybuilding is all about! Assuming that you have some decent size to work with, here are some pointers:
1.Attend at least one local bodybuilding show, including pre-judging. This step cannot be missed, it's critical! In fact I recommend that you attend 2-3 shows before you ever step on stage. You need to understand the entire process, the pre-judging, how you and should not look, etc.
2.Get help from someone that has competed before, preferably a seasoned competitor, not your gym buddy that did one show. The mistakes that an experienced competitor can help you avoid is invaluable. Even if you have to hire someone, it will be worth the investment.
3.Give yourself adequate preparation time. This will vary from one individual to the next. There are many factors involved, most importantly the amount of body fat you're currently carrying. It would be wise to have this measured to give you an idea of how much you need to lose so you can plan out how long you will need to diet for. Remember the key is to lose body fat (slowly) and maintain muscle mass.
4.Do not over diet! You must lose body fat slowly, 1-2 pounds is the most anyone should lose per week, so if you have concluded that you need to lose 20 pounds you're looking at a 12-16 week contest diet.
5.Get ripped! This is the most important aspect when you step on stage, you cannot flex fat. You must get down to at least 4% body fat. The leaner you are the bigger you will look on stage. It's an illusion, even though you will feel like you're shrinking down to nothing, you're better off sacrificing some muscle to get ripped.
6.Get everything in order early on, have a game plan. Plan out your entire contest diet in detail! Get all necessary items ahead of time:
A Tanning creams (Pro-Tan, Dream Tan, etc.)
B. Posing Trunks
C. Posing oil (not really needed if you use Dream Tan)
D. Supplements
E. Posing music
F. Routine (create and put down on paper for reference)
7.Get a friend or family member to video tape your pre-judging and the night show. Having this video is a very effective tool in evaluating yourself, not only how you looked, but your posing, stage performance, etc.
8. Learn and practice the mandatory poses! These must be second nature, on stage there will be no mirror, so once you have learned to pose correctly start posing without a mirror, see if you can get someone experienced to watch you hit the mandatory poses. No matter how good you look, if you cannot properly display your physique it will not matter.
The key to competing in bodybuilding is experience, the more you compete the better you will get. The more times you step on stage the more comfortable you will get being in front of an audience. Do not get discouraged if you do not place, or place well, at your first show, instead find out what you did wrong and how you can improve it for the next show. Good Luck!
Here are some links of articles that you may find useful:
Bodybuilding Posing
Contest Dieting
Bodybuilding Competition FAQ
Do's & Don'ts of Your First Contest
Perfect Contest Tan
NPC Rules & Regulations
NPC Contest Dates
Also, be sure to check out Natural Bodybuilder Tom Venuto's complete fat burning, muscle building system that he has personally used to win many bodybuilding competitions: Burn the Fat, Feed the Muscle | 
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