Intercourse for the first time, I don't know how to put a condom

So me and my gf are gonna have sex together for the first time soon. We've been together for a bit more than a year. We are ready for it and she's very excited about it. I want to make sure nothing happens because I've read that it could change her period. She has a 28-34 day period and it's very irregular with some missed periods. What can I do to be safe from pregnancy so I don't get paranoid??? We agreed that I will *** before we have sex, then once I get the wood up lol, I'll put on a condom and then we would have sex. Even though I don't expect to cum again, even if I would I would pull out with a condom. I know how to put on a condom and take it off properly. Any other things I can do? (Birth control pills aren't an option)