-서 is most frequently used as a causative ending. It's one of conjugational endings which always include -아/어 before itself.
It would be likely to sound complicated, but (I hope) it will be rather easier if you try to see how it works systemically. It's just different from the way English works.
One example :
A : 얼굴이 왜 빨개요? Why did your face turn red?
B : 추워서요. Because it's cold.
In the example, 춥다 to be cold, is an irregular verb. -다 is a factual ending to make the adjective verb stem into the verb(You can just say 춥다 to your close friend.) So you can figure out 춥- is the verb stem. [ㅂ] of it becomes 우, remind yourself of this fact every time you see 『-ㅂ다』-shaped verb, like 맵다, 곱다, 좁다 etc.
So you have 춥-, -아/어, 서 now. You make 춥 into 추우 because you want to use a conjugatioal ending, -서.
See the last vowel of the verb stem, if it's [ㅏ] or [ㅗ], pick -아. If one of other vowels, pick -어 up.
추우- + -어 + -서 = 추워서
When you use "because of + the noun", we use "the noun + 때문에".
There're several causative endings of course, but I think it's not too late to learn them after getting used to using -서.