You need to specify what kind of "have to" this is. i.e. I have to do my homework (informal). 나 숙제 해야되. 해야+ is how you express "have to" when it's about doing something. Hence it comes from "하다." If something has to become something, then it's: i.e. It has to work. 꼭 되야 돼. So it depends on what context. And yes, use the site Leatteair forwarded you to. I have heard many good things about talktomeinkorean.com

V+어/아야 되다 // V+어/아야 하다. EX: i have to eat --> 나는 먹어야 해요 / 나는 먹어야 돼요 i have to go --> 나는 가야 해요 / 나는 가야 돼요 to eat: 먹다 -> 먹어요 to go: 가다 -> 가요

yeah, but they are late in answer =(

http://www.talktomeinkorean.com/lessons/level-2-lesson-20/ Are you using Talk To Me in Korean series to study Korean? I love this site.