What kind of grades do you need? Pretty good ones! A's and B's are expected. In addition you will need to offer a sport or other exttracurricular activity. That will enable you to fit right into the life and community of your new school. After all, you will only be there for a year. Stellar recommendations from your current school will help.
The application process is a little different. Instead of taking SSAT or ISEE, you offer PSAT or SAT test scores. Places for PG year are extremely limited. Be sure to apply as early as you can. Need financial aid? Expect that to be limited. Always ask up front.