Summarize the following:
You don't need to get clinical or overly descriptive--no need to get into the strength of your flow or anything. Chances are he's been through sex ed and had other girlfriends (or a sister or close friend) who has mentioned it before, so it shouldn't be a big deal. Say something simple, like "Hey, I just started my period. I'm not feeling so great." You could also say "Oh, it's that time of the month," and he'll probably get it. Sometimes couples come up with cute or funny code words for when you're on your period. So when you tell him it's "shark week," he'll know what's up. Even if your boyfriend doesn't know much about the mechanics of your period, he probably knows it can be a rough time for some women. Your boyfriend cares about you, so he probably wants to know what he can do to make you feel better. If your period makes you feel exhausted and crampy, ask if he can hang out with you on the couch while you binge watch and eat ice cream. If you get cramps, you can ask him to rub your back or your belly to help relieve some of the pain. Let him know if you become particularly sensitive to touch. He might try to comfort you with a hug or other contact, and it's only fair to let him know you you really prefer not to be touched while menstruating. If you prefer to hang solo when you're on your period, you can (kindly) let him know that just letting you be on your own is a huge help. If he can't handle the fact that you menstruate, then he's maybe not boyfriend material just yet. And he's definitely not mature enough to be having sex, if that's something you are considering. At this point, guys should have come to terms with the fact that this is a normal part of every woman's life, and the best thing he can do is be supportive.
Tell him in a straightforward way. Let your boyfriend know how he can help. Think of it as a good way to weed out immature guys.