You need to mix records with someone. Basically, you need another game of R or S, link them up, choose mix records, then something'll happen (I haven't mixed records in ages). Once you're done, go to your Secret Base, and you'll find a trainer that depends on the last number of your ID (e.g. 1 = Bug Catcher, 9 = Lady ect). I think that's how you do it, although I haven't done it in ages...