This one’s got to be tough for people trying to learn English. First of all, neither word is pronounced the way it’s spelled. The s in loose is pronounced as though there is no e at the end. That is, with a hard s, like sss, rather than the z sound it so often is. The s in lose, on the other hand, is pronounced like a z.
The vowel sound is the same in both: it’s the oo sound, like in room or even – to further confuse those non-native speakers – tomb. But when do you use which? The way I remember is that it’s almost always lose that I want to use. While true in my experience, that’s probably not very helpful to anyone else.
Lose means to misplace or get rid of something. You lose weight or lose your car keys. If something is not tied tightly enough, it’s loose, like your pants would be if you lose weight.