Most receivers are already drilled and tap for scope mounts. Look at the top of the receiver and you should see four screws.
You buy a base (your choice) and rings for the scope. Now, generally you want the scope as low as possible. Why? Because the higher the scope, the less positive the cheek weld. It'll be a chin weld if the scope rides up too high (that's why some guns like the No 4 (T) or HK PSG-1 has a cheekpiece on it). The determining factor of the height of the rings is comfort (yours) and the objective of the scope (the big swelled out part where the lens is). The scope ring must be high enough such that the scope is touching the rifle.
Now, as to the scope, that's entirely dependent on your intent, budget and taste. I'd be happy with a 4 x fixed scope myself - but that's me. My favorite brand is Leupold (and most common) but you may want something different (I also have some Tascos, a Bausch & Lomb & other scopes).
