First of all maybe Bob Smith is right you might not need AAS for a first Fitness show if you in good shape, altough I would still use Clenbuterol/ECA and you might need something to help you dry out (diuretic, alotugh it can be very dangerous if not used properly).
It's very iportant that you have some gymnastics background because your routine is very important.
As for the diet when I work with my clients I use different approaches because everybody responds in a different way, so the diet has to be "customized" for you, but just to give you some tips: high protein, low carbs (not zero carbs thought), I usually dont count fat, since there is some fat in most Protein sources. It is also a good idea to flactuate your carbs in order to speed up your metabolism, for example you go 5-6 days on low carbs (about 50 gram) and than 1 day you go high carbs (about 200 grams) and lower the protein.
Best protein sourses: egg whites, chicken breast, turkey breast, fish, protein powder. Be carefull with protein powder and egg whites towards the end of your contest prep since the sodium content can be a bit high.
If you do decide to try AAS, than for a beginner Anavar is ideal choice and if used properly will give great results.
Hope I helped.