Because you are training to build muscle mass/size, only 1 set is needed. Several studies have been done comparing 1 set vs. 3 sets. 3 sets greatly improves strength, but compared to 1 set shows no added benefits in building muscle. Believe it or not, increased strength may actually constrict your ability to build muscle mass. That's why you'll see a big bodybuilder who benches the same weight as a powerlifter half his size.
For your warm up and working sets go like this:
1st set: 10 reps with a weight you could easily do for 20
2nd set: 8-10 reps with a weight you could do for 15
work set: 8-10 reps to a "sticking point"
*Sticking point: this is a part in your motion where you despite how hard you push, the weight will not budge. For example, on the bench press this usually occurs when you're arms are bent at about 90 degrees.