You're right, sweating=soldering, but sounds like you got the wrong valve. If the old valve screws on to the pipe, then you just need a screw-on replacement valve. What you want is a ball valve that takes just 1/4 turn to go full off to full on and are full flow valves. It will look very much like the one you got, but will have threads. However, if you are using screw together fittings, you must have a union in the line.