First of all, always warm up thoroughly your upper body.
If you have no equipment available: good old push ups, making sure you do each move fully, but do not let your chest rest on the floor at the end of each repetition (=complete individual move), so that your muscles are always exercising during your set (=series of repetitions, after which you can allow yourself to rest). And do the repetitions slowly.
Try two sets of 10 reps for a start, then you can experiment with 12-10, or 12-10-8. Resting between 1 to 2 minutes in-between the sets.
If you have equipment, lying on your back on a bench, and using dumb bells to do "butterfly" moves would work. You can apply the same number of reps/sets as above.
However, for maximum effectiveness, change your routine regularly (at least every two months), and try working out your pecs in different angles, adding to either of the above exercises something like dips (using two benches, or alternatively 3 chairs), and lifting up (while standing/sitting straight) free weights. Both these types of exercises will help work out respectively your the lower and upper fibres of your pecs, but should not be done on their own since each only partially trains the muscle.