You’ve gotten lots of great advice here, UWmite. I had chronic UTIs for an entire year when I was 5. Coincidentally, it was also the year I started school and refused to use the bathrooms there. I gave up Mr. Bubble, took lots of antibiotics, and had an IVP when I turned 6.
I have mostly grown out of them, but I find that I do have to make lifestyle choices (wearing only cotton undies, making sure I pee after sex, etc.) as a preventative measure. I also had a lot of trouble with them when I used the nonoxyl-9 spermicide.
Another factor may be a weak kegel muscle. Women who have a weak kegel muscle will sometimes not fully empty their bladders when they urinate. Do you ever pee your pants a bit when you laugh or sneeze? Do you sometimes get an urge to urinate, and then only a few drops come out? If so, you may need to strengthen your pc muscle with some kegel exercises. They may help prevent some of your UTIs.