AMC 10/12 is where real problem solving begins. Chenchuan is correct in saying that the vast majority of kids, who are good in math, need proper training and lots of it to be able to qualify for AIME.
I would suggest going throught Art of Problem Solving books, if you have them available, and doing old AMC 10/12 tets.

Chenchuan is right - participation in these tests and scores are important if you are planning on pursuing math or physics at a reputable university.