First pointer would be figure out if you want to add outlets to an existing circuit or do you have the space to add a whole new circuit. The standard for adding a new circuit would be a 20A breaker and 12-2 wire w/ground. I don't believe there is any certain number of outlets that can be on one circuit but if you are only talking about adding 4 or 5 new duplex receptacles, it would probably be ok to use the existing one.
Personally, I would add a whole new circuit since the existing outlets are probably on one circuit. It would be a good idea to have 2 different circuits so if you are running things with a high draw like a small mig welder and air compressor you wont be popping breakers.
As far as wiring it yourself, it is very simple. However the most important step is to insure the power is off to wherever you are working. Black is power, white is neutral and bare wire is a ground. In the panel the black wire would go to the breaker, white and bare wire to the neutral bar. The back of the receptacles are labeled for black and white, and the ground will be at the bottom. Also the terminals for black usually have gold-colored screws and white will have silver screws. When wiring from one circuit to the next, the bare wire will need to be twisted together and one leg shortened as there is only one ground terminal on the receptacle. When making the connections hook the wire around the screw so that when you are tightening the screw clockwise the wire is being tightened. In other words, looking at the top of the screw, the wire should start from the left and loop around the screw to the right.
