Hi there. First off, congratulations on being responsible, and making the decision to not only use condoms, but to find the right one for you.
Depending on where you're from, there should be a health center or something similar in your town, that offers free condoms. A lot of them will have baskets of condoms in their public washrooms, so anybody can walk in, go straight to the washroom, and take some. This is a good place to start. Whether or not there are more than one kind offered, it's a good chance to pick up a few, and test them for the right fit. Most of the time, these places will carry "normal" or "average" sized condoms, so it's a very good starting point. If the condom is too loose or too snug, you have a few options.
1) Buy some yourself, and figure out what fits you best.
2) If you have an older sibling, cousin, friend, other family member, or a parent or older relative that you feel comfortable talking to, you may want to turn to them for advice.
3) If you aren't comfortable talking to a friend or family member, there's still the health care center, if you have one. Go in and up to the front desk, and all you have to say up to that point is something along the lines of "Hi, I was wondering if there is a health care nurse available to talk to right now." Anotehr alternative is a pharmacy, as well. I know it can be awkward at first, but these people are there to help, and they help all kinds of people, from teens to older people, usually several times a day, with all sorts of sexual and other health issues... they'll never look down on you for your decision, in fact, they're more likely to also congratulate you, for making such smart choices. Explain to them that you're sexually active, but haven't had intercourse yet, and that you're wondering what can be done, on your part, to keep yourself and your partner safe. They'll do their best to help in any way they can.