Skeetendo

first: which game you mean?
there's Pokemon Red, blue, yella, gold, silver and crystal.
the methods for pokemon insertion are pretty different.

second: look at the FAQ in this site, you'll find many useful threads for GenI\II hacking.

last: there's not always a tool for eveything.
most of the changes should be done with hex-editing, debugging or disassembling - theres' not yet the "magic tool" that makes automaticaly every changes a hacker wants in his hack.

the most you get knowledge about hacking and the most you practise, the most you'll be able to hack games.

anyway remember there's a search function.

This speech is also for every new user that is entering the hacking, in one word, "try".

If you want to change Pokémon sprites use AGIXP. If you want it look around Tools/Resources. For pokemon sprites search Google