What do i need to travel to thailand?

Im travelling to thailand in january with my boyfriend (he's thai). But im not sure what i need to do before i go. He says needles and pport. but he doesn't explain things very well. so i was wondering if anyone had a checklist of things to make sure i have, like pport, visa, go to the doctor, immigration stuff?? i need a straightforward answer explaining everything i need before i go. We're going there for 3 weeks so i dont think i need a visa? Please help.