You have two basic ways to go... 6mm or .177
You can go with AirSoft (6mm plastic BB's) or .177caliber pellets/BB's. I have both and they each have their pluses and minuses.
You can shoot either one in your garage. AirSoft will do less damage indoors, you could actually shoot it in your house our apartment without many worries. I wouldn't do that with .177
AirSoft has a lot of variation. You can get single shot, spring guns that shoot around 150fps (feet per second) and bottled gas models that shoot up to 310. To give you some idea, at 150fps, the BB will not even penetrate the cardboard used for 12pks of cola, whereas at 310, it will penetrate a soda can and go all the way through it if you are close. There are a few CO2 AirSoft guns that go up to 350 or so. For CO2, .177 handguns, 450fps would not be unusual and being a smaller projectile, it will put quite a hole in some targets.
Probably the most fun shooters are the ALL METAL AirSoft guns that are very close to the real thing in appearance and operation. They are blowback and add realism to firing the gun. If you have a real Glock, Beretta, HK or 1911, you can probably find one that is just like it. Here is a link to a Beretta that is rated as one of the hardest hitting and most accurate. There are other good guns if you want to do your own research, I am just trying to make it easy.
http://www.airsplat.com/Items/GP-KJW-M9-MIL.htm
You will not believe the realism the first time that you see and hold this gun. It also field strips.
If you like the 1911 style, you will like this one. It comes with a bulk gas type mag but you can buy a special 12gm CO2 mag for it so that you can use either bottled gas or 12gm cartridges. One really nice feature of the WE 1911's is the adjustable Bowmar sight.
http://www.airsplat.com/Items/GP-WE-HiCapa-V5.htm
If you want a .177cal, there are many available and too much to cover here. I recommend the Gamo brand because I prefer CO2 to pump and I prefer repeaters over single shot. I want action, not loading and cocking on every shot. They are pretty good for the money.
http://www.airgundepot.com/gamo-air-pistols.html
If you want pump or single shot, I am not the one to give advice here.
For holsters, check out that AirSplat site or any AirSoft dealer. I bought a cross-draw tactical vest today from a local AirSoft dealer and it is the same thing that I could get at the gun store.
BTW, I had a very bad EAA Anics pistol so be warned.
Always wear eye protection!!!