A couple of books first. Paletta's The New Marine Aquarium and Kurtz's the Simple Guide to the Mini-reef Aquarium.
What's your fish-keeping background?
Then, start asking questions. 38l is very small for a Marine tank, especially if it's your first, which I garner that it is. What kind of tank is it? Is it an all-in-one tank (with filter lights, and everything) or is it just a standard 38l?