Many possible answers. Simple one first.

Open an assembly file. Bring in the most important part (e.g., if you were assembling a ball bearing assembly, start with the inner race). Place that with respect to the origin and principal planes and fix it. Immediately fully name and document that part. Add any notes that you might require.

Next, add the mating parts, mating them to the first part as in real life. Spend a lot of time on mates. Now, check the mates again. Again, document that part.

And, so on.

After you've gotten past about ten parts per assembly, buy Matt Lombard's SolidWorks 2009 Bible and follow his methods. The minute that book arrives in the mail, erase from your mind all my instructions. Matt is either the best or second-best SW guru there is (I don't think I could choose between Matt and Anna Wood).