rg - along with good GPA, GRE scores (general and subject), you will also need a decent personal statement to get into a good program. In your personal statement, you could include why you are interested in the subject - particulary which aspect of it. You can also mention what you want to do after you complete your grad school (Masters/PhD). You should also mention any REU (research for undergraduate) program, if you have done. Or, any research experience as an undergrad.

In addition, if you are interested in joining a particular lab, you should also write to the professors directly.

Good luck.