whoa there!
first, there are a TON of differences between the two churches. I would really really really suggest you NOT convert for a boyfriend, especially at age 15! I know it may not seem like it now, but, most likely, in 2 years you will no longer be a couple, but you will still have your relationship with God.
MAJOR differences (and I'm not an expert on presby's so help me out here)
1. Sola Scriptura. Presby's, and all protestants believe in only the bible as the source of beliefs and doctorine, while the RCC (Roman Catholic Church) teaches that the Bible, and the teachings of the church are the guide for believers lifes.
2. Sola Fide. Protestants believe that your salvation is based solely upon your faith in Christ, while the RCC believes that it starts with faith in christ, but is also dependent on how you live your life.
3. the Eucharist. The RCC teaches that the body and blood is really and truly present in the Eucharist, while presby's see it as only a symbol used to remember Christ.
4. Sacraments. Presby's don't have them....
would you like more info on any of these?