The question is really more a matter of what you can afford than what you NEED. You already have most of what you NEED which is a gun and magazine.
1. Decent quality AEG
I'm sure the Echo1 club will chime in shortly here, but here's my issue. While yes, some Echo 1's last a very long time, for the most part they are very hit or miss. This becomes more of an issue once you have to worry about upgrading/replacing parts. You will find issues with parts not fitting properly are far more common on E1's and other Chinese guns than they are on Taiwanese and Japanese guns. Secondly, E1's also shoot too hot out of the box in most cases which means right off the bat you either have to take the gun apart or pay someone to do it. That said, Gearbox will do it for you for a reasonable cost. Here's my recommendations, brand wise.
KWA - They start at the higher end of the price bracket ($350) but I've yet to own an AEG that performed better out of the box than my KWA SR-10. This is from someone with 30+ high end AEG's.
ICS - Anyone who has been around any length of time knows I like my ICS's. They are by far the easiest M4 platform to work on, they are extremely reliable in stock form, and they are reasonably price. Proline models start at $250, Sportline models start at $150
Ares - Extremely nice externals (most of the time), very reasonable prices (most of the time). I personally however, haven't been terribly pleased with the performance of any of the 4 Ares guns I have owned. Their assembly process is entirely devoid of any sort of QC.
G&G - Their Combat Machines (aka Sportline) models are very hard to beat for the price and look pretty nice. Unfortunately they seem to have some QC issues.
Now that I re-read your post, I realized you already purchased the gun, so ignore that first part. However, if you end up purchasing a second gun, it still applies.
2. Battery
I'm pretty much with Axis on this. Even if your gun of choice comes with a battery, get one GOOD battery and a GOOD smart charger. If you stick with the sport, both of these will last you years. If you drop out, they can be sold pretty easy. Having a spare battery never hurts, but unless you are really trigger happy, one battery can get you through a game. Expect to spend about $100 on a good battery/charger combo. If you do any RC cars/planes your existing charger will work. Battery may work as well. As long as you are aware of the risks before hand, I recommend going with Lithium Polymer (LiPo) batteries as it's cheaper to start with them than to switch to them later. The prices have gotten low enough that you can get a good LiPo charger and battery for about the same price as NiMh/NiCd.
3. Good Eyepro
Do not skimp on this. Regardless if you go with Goggles or glasses, make sure it's rated. You can get a quality pair of goggles for $20-$30. A lot of fields require sealed eye protection and indoor field often require a full face mask, so keep that in mind when choosing your eye protection.
4. Gear
I'm going to have a little different view point on this than the other recommendations. Hold off on gear until you know you want to get in the sport and are happy with your platform of choice. I haven't seen anyone address where you are playing but at indoor games and some of the smaller games, you can get by with no gear at all. I've shown up to some games where I was one of only 3 people actually wearing a vest. At most get a cheap load bearing setup with the appropriate pouches. You can end up spending hundreds of dollars on gear once you get into it. No point in buying a $150 vest only to decide in a month you don't like the sport or need different mag pouches. IE, you buy M4 mag pouches and then decide to buy an AK. Hydration is the only piece of gear I would say is a necessity if you are playing anywhere other than indoors. I've played games before where my gear consisted of a belt with a canteen and drop pouch. I just shoved a few mags in my pockets. No, it's not tactical and you'll probably get laughed at but it works. If you've got the money to burn, by all means, buy some gear but it's not a REQUIREMENT to play at an airsoft game in most cases. Even after 8 years of playing, I'm still running with cheap gear. I personally don't see the reason to spend $200 on a vest when my $50 vest does everything I need it to do.
5. Uniforms
I'm putting this in a separate category from gear as it's a little harder to get by without but once again, not impossible. For many games, you CAN get by with tan or green colored clothes. However, BDU's (especially Woodland) can be had for pretty cheap so there's no reason not to have some. Multicam is my preferred pattern since it blends just about anywhere, but it's a bit pricier since you can't just pickup some surplus gear. As with the gear, I'm not into the whole gear whore/impressionist thing so I just go with whatever works. If you want the good stuff or want to replicate a certain look, more power to you, it's just not my thing.
For both gear and uniforms, Army Surplus is a good place to start. Gearbox also has a pretty good selection of "entry level" gear as well as some surplus uniforms which is what I run with anyways.
6. Footwear
This somewhat goes in line with gear. While boots are good, I wouldn't say they are necessary unless you are playing in the bigger out door games. If you are playing indoors or at smaller games, you can get by perfectly fine with a quality pair of sneakers. Once again though, if you have the money or if you are playing in rough terrain, pick some up. I've had the same pair of Bates boots for a few years now and love them. I'd pick up a good pair of boots before gear as gear is a convenience, boots are for safety.
7. Radios
Once again, unless you are going to be hitting up the big games right off the bat you get by without a radio with no problem. Most of the vets run ICOM F4S radios but you can certainly get by with a cheap Motorola.
8. Mags
As with the previous points, this isn't something you should be stressing about for your first game. You gun will include at least one high capacity mag. While they are frowned upon (I usually just give mine away), it's pretty rare that you will be turned away from a game because you only have a high cap. The only games that will do that, mention that in advance. That said, a box of MAG brand mid capacity mags will run you about $70 and then you are set either way. Just keep in mind you will want a good speed loader if you get mid/low capacity mags. Get an M4 style speed loader as they hold more BB's and fit in M4 mag pouches when you end up getting gear.
9. Side arm
Excluding times when I am running with a sniper rifle, I can count on 1 hand the number of times I've HAD to use my side arm. That said, I recommend AEP's for a sidearm. They are cheaper and more reliable than most gas pistols. They are easier to maintain and less of a hassle to use. That said, they are much more lacking in the realism department. Cyma CM.030's (Glock 18 AEP) can be found for under $100, sometimes as low as $50. I do recommend getting Tokyo Marui brand mags rather than the Cyma mags however. That said, they are still cheaper than magazines for gas guns. At CQB games, I'll often run with just an AEP (either MP7 or G18) because of how light and maneuverable they are. Unlike gas guns, they also function in pretty much any temperatures.
That's my thoughts anyways.
