Bench Press :

Bench press is a compound exercise but obviously better then push-ups. If you are serious into the pecs business you must do them.

How to do Bench press:

You should lie on a flat bench with your feet on the floor for balance.
Your hands should be far enough apart that your forearms point straight up (perpendicular to the floor).
Start with the bar at arms length and then lower it towards your chest.

Make sure that when lowering the bar your elbows point out, working the whole chest.
You should bring the bar down until it just touches below the pectoral muscles(The weight should be almost touching your nipples when down).
Never bounce the bar off your chest, this is poor form and it can lead to cracked/broken ribs and/or bruising of the heart.
Return the bar to the starting positio

Its similar to the flat bench press, just the bench is a little raised (inclined).

The basic difference is that unlike flat bench, you lower the weight to your upper chest . This works more on your delts and upper chest.

Hows much your delts get work is proportional to how inclined you are. Normally 30-40 degrees is ok.

Make sure that the bar doesn't drift forward, lowering to farther down the chest. If this happens you won't be working the upper chest as much, the shoulders will be doing more work.

Here the bench is adjusted downwards. The angle is about 23-30 degrees.
Its a compound exercise and is very good for lower pecs also the front deltoid.
Bring the bar down you should bring it down to the lower part of the chest.
Make sure that the bar doesn't drift backwards. If this happens you'll be working more of the pectorals rather than focusing on the lower portion.

In all exercises, you needn't lock your arms, otherwise you won't get all the benefit.
Try very low weights if you are beginner, to get the feel of the movements.
Also in the last few reps or if you've never done them before, you may find that one arm is working more than the other. I mean, your barbell is not parallel to the floor. Avoid that as it may hurt you
In all the exercises, keep your wrist straight.
Also, do the negatives with more or less the same pace as the positives (that's important and not doing so a very common mistake).
Another sign of bad form is raising your shoulders. Don't raise them.
In all the exercises, go up slowly, wait for a second and than come down with the same pace. The more you narrow your grip more you work your triceps. Also, try to actually feel the muscles working.