all my replies so far tonight have been really sarcastic and assholey so in the interest of gaining back some good karma I'll actually try and help.
My advice is not to use steroids. The fact that you can't gain muscle without them means you will gain very little when you use them, and then lose it all when you stop taking them.
There are people who for some reason claim that you lose all your gains when you stop using steroids. This is not the case, however think of it like this: If you were to build 10lbs of muscle on a steroid cycle, you need to then be eating enough calories to sustain that muscle. If you can't eat enough to gain weight at the best of times, you sure as shit aren't going to be able to choke down enough food to keep the extra muscle (and it's unlikely you'll build all that much in the first place if you're not eating enough).
So hold off on the steroids until you are in the habit of consistently eating enough to gain weight. Once you've mastered that, feel free to do some roidin