This answer is a bit tricky. Typically, though, yes—since will very often indicate a premise which then introduces a conclusion.
Ex: Since I want to go to law school, I should study hard for the LSATs.
However, at a higher level it's possible to encounter statements where since does not factor into the premise-conclusion structure. This is why we always are careful to say that "since" is one of many HINTS, not rules.
Ex: Since antiquity the Earth has orbited the sun. This is because our star's immense gravity pulls the earth around it.
In the above example, sentence 1 is the Main Conclusion, and "since" has no bearing on that
