What to do when you think you have a UTI coming on? ?

you can ask your mom or see a doctor

Go to a doctor.  BTW, you can't drink enough cranberry juice to clear the infection.  

Keep drinking water and make an appointment. The thing with UTI's is, they never fully go away on their own. A doctor will prescribe antibiotics to flush it out completely. You can also drink PURE, 100% cranberry juice. I did this, mixed with green tea and water, and it was VILE ( I hate green tea), but it worked until I got to the doctor's. You have to drink enough to keep peeing and flushing it out without kicking your kidneys into overdrive, so drink enough to pee a lot. A UTI is going to trick you into feeling like you have to pee with nothing to show for it, so make sure you have enough in your bladder for a full pee, at least.

I've heard that cranberry juice works. But honestly, just go to the doctor. You're not going to overcome an infection like this without antibiotics.