I'm going to echo the sentiments already expressed but try to be a little more gentle so that you listen to my advice rather than sack up against some kind of personal attack...
Assuming you have normal testosterone an GH levels for a 20-year-old male, you're already primed to put on plenty of muscle naturally. Whether or not you take steroids, you're going to have to eat a shitload to put on 20 or more lbs of muscle and you're going to have to keep eating that way to keep it. It's simple physics, you can't pull that mass out of thin air; you have to eat it.
At 5'10" 140lb, you are nowhere near your genetic potential to hold muscle. And if you knew how to eat to grow, you wouldn't weigh only 140lbs. Building and maintaining a muscular body takes knowledge and dedication. Diet and Training are the big levers while steroids is just a fine control tuning knob. At this point in time steroids is a waste of money for you.
If you seriously want to add more muscle, start eating at least 3,000 cal/day. At least 200 - 250g of it from protein, about 900 calories of it from fat, and the remaining 1200 calories from complex carbs. Time your carbs for first thing in the morning, and before and after workouts. Avoid them in the evenings and just stick with vegetables (no bread, potatoes, or pasta). Eat every 2-3 hours, throw a meal-replacement protein shake in where you have to but no more than 2 a day.
Research on the internet for a 5X5 workout program and stick to it 4 days a week for 16 weeks. Then come back and post up some neat pics of all the muscle you've put on thank everyone for talking you out of a steroid cycle that you didn't need.