Correcting your posture is an ongoing process. You need to read up on correct posture, and then notice throughout the day when your posture begins to slump. Tuck your pelvis slightly and tuck your chin slightly. Your ears should line up with your shoulders, your shoulders should be back slightly, with your chest up (not exaggerated, of course), and your hips should line up with your shoulders.
You could also see a chiropractor for an adjustment, and have him or her help you with the correct posture. But it will still be a practice of noticing it several times everyday. It isn’t something you can just correct with a few exercises and then you’ll have better posture.
Good luck! Better posture really helps both your attitude and your body. :-)