Att & Som

Roughly translated, both att and som means that.
But they are different in one way,

Att is used in questions or statements that refers to possible or definite outcomes. "Is this going to happen?", "Will he make it?", "This is what we'll do." and so on.

Tror du att han klarar av det?
Do you think that he will make it?

In parts:

Tror du att - do you think that
han klarar av det - he will make it


Som on the other hand is a word to use when you're talking or asking about a subject, a place or an action, but not any kind of question. For example, questions with a request doesn't apply: "Could you fetch me my towel?". Instead, sentences like "There's that girl who won the game last night", and "Is that the dog who bit you?", "The man who would be king".
I hope you understand what I mean, because it is very difficult for me to explain.

Är det den där skivan som du så gärna ville ha?
Is that the album that you wanted so badly?

In parts:

Är det den där - Is that the
skivan - album
som du - that you
så gärna ville ha - wanted so badly

But the words kommer att together means will or going to, gonna:

Jag kommer att köpa en bil idag
I will buy a car today or
I'm going to buy a car today

I hope all this made sense!
Til next time, see you!