kind of a baby

I am really new to this sex life. I don't really know the difference between a UTI, STD, and STI. I also do not know what is curable and what is not. Ive learned these things but my parents never really talked about it. What causes UTIs? How do you get rid of them? Do you HAVE to take medication? How long do they last? What's the difference between the 3? Ive tried to have these conversations with my doctors but because I come from a conservative home and I was a virgin at the time, I always felt scared to ask. Now it is a little harder because I am in school and our health center is a little different. I am really nervous about this whole uti thing. I didn't pee after sex so it may be from that but catching something is really one of my biggest fears.