First spending some money on GOOD gear is worth the investment. I have cammies that have lasted nearly four years now. Next get gear, a tactical vest is well worth the investment because it sets you far apart from everyone else sticking mags in pockets. It also will make life a hell of a lot easier cause it gets your weight onto your midsection instead of pulling on your clothes.
Get mids for magazines, mag AK mids go for under ten bucks a pop and they are normally 140 round capacity. 6 of those is all you should need, and they will work a hell of a lot better then ANY high cap.
Avoid messing with your gearbox or anything else until you have the cash to get good upgrades. Throwing shit into your gun just gets you more shit.
Also don't forget to plan to buy a few batteries, decent bbs, and a GOOD sling. Cheap two point slings (like the ones sold with a lot of guns) don't help you at all. They don't let you carry your gun any better, nor do they create a more stable platform for firing (when you sling in) and if you rely on a shit sling it is going to end up with your gun getting dropped.
Clothing
Gear
Mags
Upgrades