how can I train only my chest when i bench press with barbell or dumbbell?!

It depends on what you consider to be the chest. You cannot isolate the pecs. Period. There is no motion that involves the pectorals without involving other muscles. Usually the ancillary muscle is the frontal deltoid.

The best pec isolation will come from flys which have to be done with arm-independent resistance (dumbells or machine). You can't do flys with a barbell because you must do a press and that will involve the triceps.

I don't know why anyone would want to isolate just the chest. However, if you want to work the pecs harder you can train your mind to concentrate on pectoral contraction. The exercise only gets you in the "ball park". It's the mind that actually controls what muscles are doing the work. Then exhaust the arms before doing the flys so you'll be less likely to use them to "cheat". Do that and you'll get the best possible pec isolation.

Good luck and good health!!

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If you are doing bench presses with a barbell or dumbbells, and using proper form; you are training your chest.

It's true that your triceps will get some work as well, but why would you want to eliminate that? The bench press is a compound exercise (like the squat), and as such it's going to work multiple muscles at one time. There's nothing wrong with that at all.

when you are working out the chest you are training it directly... the exercises are mostly using the chest... indirectly you are using your triceps and shoulders....

which is not a bad thing at all... you should also train the chest with bench press at 3 different angles...

incline bench, decline bench, flat bench
and also try doing a pec dec machine
and if u want some dumbells.. that will be the best workout