Learn Russian: Раз ты здесь, помоги мне.

In colloquial Russian, раз acts as a conjunction meaning since, given that, or once. It introduces a known fact or condition that makes the second part of the sentence logical.

Could I use так как or поскольку instead of раз?

Yes, they also mean since, but they belong to a more formal register. Раз is the most natural choice for everyday casual conversation. Also, remember that you generally cannot start a sentence with потому что, which makes раз a very handy alternative.

Why use the perfective imperative помоги rather than the imperfective помогай?

You are asking for a specific, one-time result right now, which calls for the perfective aspect. If you used the imperfective помогай, it would imply an ongoing action, meaning help me out on a regular basis or keep helping me.

Can I change the order of the clauses and put раз in the middle?

Yes, помоги мне, раз ты здесь is completely natural. Starting with раз, as in this sentence, just puts slightly more emphasis on the condition by highlighting the fact that the person is already there before making the request.