Barbell bench presses will develop your pectorals as well as your triceps, shoulders, and back.  It tends to be one of those exercises people use to judge strength (How much can you bench?).  Generally speaking, one should be able to bench their weight, but of course, things vary.  It is a basic movement from which a number of other exercises are derived.

Starting Position

Load the desired weight on a barbell and put on an exercise bench.  Lie on your back on the bench underneath the barbell and grip the barbell slightly wider than shoulder width with an overhand grip.  Place your feet either firmly flat on the floor on either side of the bench, or bend your knees and place your feet together on the bench.

Performing the Exercise

Push up on the bar lifting it off the rack.  Lower it under control to your chest either just touching it or just above it.  Do not let it bounce off your chest.  Concentrating on your pectorals, use them to push the weight back up without fully locking out your elbows.  Lower again and repeat for the desired number of reps or time.

Tips

Breathe and don't be stupid.  First and foremost, if you are using a weight you are not sure you can handle, have someone spotting (ready to lift it off of you if you get into trouble).  Don't be too proud to ask someone.  Second, breathing is important.  It maintains a pressure equilibrium in your body and ensures you don't blow all the blood vessels in your head.  Breathe in as you lower the weight and breathe out as you push the weight back up. (feel free to grunt and moan as well ) As with all exercises, the more you concentrate on the contraction, the more you are going to get out of it.