The most commonly used AAS in medicine are testosterone and its various esters (but most commonly testosterone undecanoate , testosterone enanthate , testosterone cypionate , and testosterone propionate ),  nandrolone esters (most commonly nandrolone decanoate and nandrolone phenylpropionate ), stanozolol , and metandienone (methandrostenolone).  Others also available and used commonly but to a lesser extent include methyltestosterone , oxandrolone , mesterolone , and oxymetholone , as well as drostanolone propionate , metenolone (methylandrostenolone), and fluoxymesterone .  Dihydrotestosterone (DHT; androstanolone, stanolone) and its esters are also notable, although they are not widely used in medicine.  Boldenone undecylenate and trenbolone acetate are used in veterinary medicine . 
You should take protein and creatine as your key supplements or a Weight Gainer product if you want to gain maximum mass. Take a whey protein shake 2 to 3 times per day with one serving being just before your Halo immediately after training and take creatine monohydrate powder or a creatine tablet (like Creakic or Creadex). Start off taking 4 servings per day for the first 5 days and thereafter take 1 to 2 servings per day (2 on training days and 1 on non-training days). To gain mass at the fastest rate, you can either do the Nitro/Cell stack (Nitro 3 times per day and Cell-Tech twice on training days and once on non-training days - always take one serving of Nitro immediately, followed by Cell-Tech right after training) OR you can use a weight gainer like Mutant Mass (along with powdered creatine) or Mass-Tech or Muscle Fuel Anabolic (which already contain creatine). If your diet is good (high in calories and protein), go with the Nitro/Cell stack, if your diet isn't so good, go with one of the mass gainers - start off with 2 to 3 scoops twice daily and increase until you are gaining the desired amount of weight per week (1/2 to 2 kilos).
Anabolic supplements are 100% legal (in the USA), unlike anabolic steroids that have been banned. They are quickly becoming more popular than pure anabolic-androgenic trainers, bodybuilders, and athletes are now much more informed about what pure steroids do to their bodies and how scary they can be. In fact, they can even be life threatening, causing people to look at alternate means. Instead, legal steroids and supplements are chemical compounds that have been made to deliver the same type results you would get from pure steroids.