At 20yrs old you don't need to start cycling, you have enough hormones naturally to produce really good gains, concentrate on your diet and training program and build a good solid base to work from, if you do this correctly over the yrs you will be in a far better position to build and maintain when you do start cycling,

Use what you have got at your age and don't risk damaging your own hormonal system up, which does occur in alot of bb's who start in their teens and early 20's,