Originally Posted by kalnaren
There are a lot of things to consider. First and foremost, overall build quality. As mentioned above, Tokyo Marui has excellent quality control and manufacturing. Their guns are about the most reliable on the market. Their FPS is lower than most brands, but FPS isn't everything.
A very good inner barrel and good mechbox internals (gears, piston, seals, etc.) will have quite an impact on performance. For example, a well made gun shooting 320 FPS will outperform a poorly made one shooting 400 FPS.
A lot of performance also has to do with BB weight. .20's are probably the most common, but heavier rounds such as 0.23g, 0.25, 0.28, 0.30 and so on all have advantages and disadvantages. Generally a heavier round will increase accurate distance, but decrease overall velocity. It becomes a bit of trial and error to find what works best in which gun. For most guns between 300 and 350 FPS 0.25g works well.
Generally, as mentioned, anything from Tokyo Marui, Classic Army, and ICS are very good places to start. G&P, STAR/Ares, and VFC are higher end, but make very nice very good quality guns. G&G isn't bad but their internals aren't always good and generally need some preventative maintenance. JG is a brand that's on the low end and, to an extent, make good starting guns. The issue with the cheaper brands like JG is that the quality control isn't always there and it's possible to get a gun that will fir 1,000 rounds and break as it is to get one that will go 10,000 rounds without issue.
Walmart guns are not airsoft. They're not even close. A decent airsoft AEG will cost you $350-$450. The commonly accepted number to get into airsoft is $1,000, though you could concievably do it for less, but not under $500. After you buy the gun you still need batteries, a good charger (I can't stress that enough), magazines, and gear if you actually want to play.