theres codes out there for gameshark that are like "replace xx with the number and u will catch that pokemon" can anybody tell me how this works? do u have to be in grass when u do the code?

Yes, I believe that's how it works. You turn the code on and you then need to have an encounter in order to find the Pokemon you've put in (by XX they mean to use a list of Pokemon that each have a separate value, like AA, AB, AC, etc).