mckenny99 has covered the basics. For practice, the least expensive ammunition will suffice, although I recommend you avoid steel cases such as the Tula brand. Practice ammo normally comes in a box of 50 rounds and maybe labeled FMJ (Full Metal Jacket). You can often get a box of 50 9mm FMJ for under $15.00. Defensive ammunition is often available in boxes of 20 or 25 and probably costs close to $1 per round.

Be sure that you get the correct ammunition for your handgun. If it is a 9mm, there are several different 9mm cartridges. What you want may be called 9mm Nato, 9mm Parabellum, or 9x19. All the same.

.45 caliber is easier because there are two common types of .45 ammunition. 45 ACP is usually for semi automatic handguns. .45 Colt is for revolvers.

Good luck, and don't hesitate to ask a lot of questions here. Lots of experienced shooting and instructors will be more than happy to assist you in any way we can.

And welcome to the Forum.