H.W.
Cramps can be awful. If you can, take a good multi-vitamin. That helps for some.
As One Perfect One said, a hot water bottle can help if you are at home, or a heating pad. If you need to go to school, Thermacare makes a great heated adhesive warming pad which attaches to your underpants. I have used these when having bad cramps and needing to go to work. They aren't the most comfortable (hot) but can be helpful if you need to be somewhere.
Ibuprofen is a good pain reliever. You can take ibuprofen and tylenol (acetaminophen) together in reasonable doses; do not take any N-SAIDS (like Aleve and Advil) together. Stay within the reasonable dosage.
Your dad was once in a relationship with a woman, your mother. I think guys do best when we give them facts in an unemotional way. When I got my first period, my mom worked out of state and my stepfather was the parent to call, otherwise it was long-distance. I called him and told him exactly what I needed, brand name and exact specified wording on the product. (this works with my husband, by the way, and he's a pretty cool guy about it). Talk to your dad about getting Midol or the previously-mentioned medications and whatever sort of menstrual hygiene items you need. He's aware of menstruation. Here's a piece of wisdom-- guys are fixers. Whatever is wrong, they just want to fix. So if you can be straightforward with him about what you need, the more satisfying the outcome for both of you.