If you're limited to $500 at startup, you may want to focus on the essentials and then gradually add more stuff as you get more funds.
You can get out to many games with just some basic pieces of kit:
1. Good boots
2. Good eye protection - You could start off with a cheap paintball mask, or go for a decent pair of ballistic goggles if you prefer them. Just avoid mesh masks and other cheap knockoffs.
3. A decent starter gun (for an M4 platform,a King Arms, Classic Army or G&P would be suggested the most), plus some mid-capacity or high-capacity magazines
4. A decent battery and charger - If you are planning on using a lithium polymer (lipo) battery, a good charger is absolutely essential. Lipos are small but hold a lot of charge. They require a bit more care than regular NiCd or NiMH batteries. A decent charger will be able to adjust charge-rates and such to suit different batteries.
This is pretty much the minimum you need to start. Many people start off attending their first game with a gun, one hi-cap 500rnd magazine, goggles and boots. Wear comfortable pants, a tshirt and any kind of gloves and you're all set.
This is a good idea because when you meet a group of airsofters and see the different kinds of gear everyone is rocking, it'll help you get a much better idea of what you want to do. You'll also get to touch and feel different guns and see which appeal to you. You'll almost certainly want a side-arm after your first game. Most pistols are gas-powered and its a good way to introduce yourself to gas-powered guns.
H&K G36c (KWA) / Glock 18c (TM) / Kimber LAPD SWAT Custom (TM) / Mossberg M500 (ACM)