I'm glad that you're asking. I'm assuming that you know in your heart that it's not a great idea at your age. For one thing, sex is always better with a long-term, commited partner. Your 13 year old boyfriend may not even BE your boyfriend by the time you read this. Right now you are both just starting into puberty and won't be finished into your late teens.
Your hormones are just starting up now, so it's perfectly natural to be interested in sex, but that doesn't mean it's a good idea. There are so many potential consequences to having sex and honestly speaking, at ages 12 and 13 you can't possibly be emotionally or otherwise equipped to deal with them.
I would suggest that instead of having sex with your boyfriend, you experiment with self-masterbation to satisfy your curiousity about sex. Masterbation is safe, natural and disease free with no worries of the possibility of getting pregnant. It's also a good way to get to know your body and pay attention to the changes that are happening to it. You have at least 5 years ahead of you before you are physically an adutlt. Don't rush into having sex. Trust me, it will be there waiting for you when you are ready and have found the right partner.