How can I have a better posture!? I get tired quickly when i try to straighten up

pushups and situps work the right muscle groups. work on your stamina.

Yoga works on posture a lot, also build up your ab muscles.

make it a habit quickly so your used to it

Strengthen your back and abs. And stand/sit a little straighter all of the time. Then, as you get used to it, stand/sit even straighter.

Push ups

Did you know by maintaing straight posture, you are contracting your abominal muscles, which is kind of a work out? You burn calories by maintaining posture. Check it out on wikipedia. It is hard to always be conscious about your posture because of how tiring it is. My suggestion is that before you start the day off, you hang on bar for 3 sets of 1 rep for 1 to 2minutes each. Throughout the day, when you find yourself drooping over into a hunch again, just straighten your back out. When you sit down, always sit with your back perfectly perpendicular to your thighs, or when you have a back rest, capitalize on it. GOOD LUCK!

just run every day for a 30 min period then the next day add 5 mins and so you wont get tired quickly i tried it

i agree with this, sit up straight as much as possible, untill it becomes a habit. When i mean sit up i don't mean arching your back all the way in, but mainting a straight lined posture.

Make it a habit until it becomes completely natural for you.