Lesson number 1: Use the crosshair.

Notice the + stuff in the middle of your screen? That's called a crosshair. When you move your mouse it will move together. Point that towards the enemy and press FIRE. Shoot in short bursts because a sustained burst will throw off the crosshair. And remember - the bullets only hit where the crosshair points, so you need to practice using it.

Lesson number 2: Use cover.

Because your character is not in god mode, learn to duck around corners and find something to hide behind if enemy firepower is too overwhelming. That is the basis of ranged combat.

Lesson number 3: Kill more, loot more.

Every person you kill drops ammunition, and therefore to sustain your ammunition levels just make sure you spend less ammo killing the other person than he does dropping them. For example, a typical raider might drop 50 bullets per corpse. Try not to spend more than 50 shots to kill that bugger then.

Or if all 3 don't work, click on this link: Fallout 3 Guide