whats the best way to correct poor posture with out going to a doctor (I was by a doctor he told me I don't need a brace & he gave me some exercises that didn't help) did anyone do or knows someone that did exercise that effectively corrected he/her posture?

You may want to consider a visit to a chiropractor and be sure all is well physically to start with. Exercises may help, but as with anything you want to change, it will take time... don't expect results right away.

Being aware is going to be the best thing... focus a great deal on correcting your posture when you walk, stand, and sit... shoulders comfortably back, lift ribcage, chest up. It may help to picture your spine attached to a pole that holds you upright or that you have a string attached to your head that is pulling you upward. Focusing on it for several weeks will help it to start becoming automatic. It will also help your muscles retrain to better support you in the improved position.
You can get a brace on your own as well without having to go through your doctor. It might be helpful for awhile so that you can better tell what good posture feels like and then you can better recognize when you start to slouch or slump.
If you want, you might also ask friends and family members to politely remind you if they happen to notice poor posture (might be wise to give them a time limit, such as reminding you for 3 months, or you will keep hearing about it! )