Instructions
The Basics
Read the markings on the gun. This will tell you what type of ammunition to use. Do not use anything other than the ammo intended for the specific gun.
Go to your local outdoors stores and gun retailers. They often carry a wide range of pistol ammunition, and the employees at the gun counters usually have a wealth of knowledge that they will gladly share with you.
Talk to fellow shooters. Gun people love gun talk, and the more experienced shooters will surely have opinions about their favorite pistol ammo.
Choosing Ammo for Defense
Pick ammunition with low muzzle flash. Also, a hollow point bullet has better stopping power.
Take it to the range. Make sure it feeds and ejects properly, and that it doesn't jam. Reliability is a necessary quality in ammo intended for personal defense. If you have problems, clean your gun and try another brand or type of ammunition.
Replace pistol ammo every few months. Use the old ammunition at the range, and fresh ammunition for defense.
Picking Ammunition for Target Shooting
Choose something cost-effective for everyday use. You will go through a lot of ammunition at the range, and especially with large-bore pistols, the bill will add up. Remanufactured ammunition is OK as long as it doesn't jam or fail. Many retailers offer bulk ammunition for a reduced price, both at the store and on their websites.
Check the rules of your range. Some ranges have strict lead requirements, and only allow certain types of ammunition. Some require you to buy ammunition at the range to ensure compliance. This certainly limits your choices of ammo, for better or worse.
Read up on the ammunition. Shooting magazines will compare ammo for velocity and accuracy. If you are a competition shooter, you may want to spend the extra money for pistol ammunition you know will hit the mark.