most guns from all the reputable companies nowadays are made to a standard that means they can all offer more or less the same features.
absolute standard features you should look for are: clamping feedneck, on/off asa, electro-pneumatic design (including eyes), easily user serviceable, adjustability (trigger, board).
if they don't have those features they generally are not worth buying unless you are buying a special item like a pump gun or something (which you probably are not at this stage).
like i said most companies are making good standards of markers and you can start looking for specific features that appeal to you.
personally the companies which produce my favourite guns are planet eclipse, vanguard, empire and dye/proto. do your research on the guns produced by them and depending on your preference to style and features as well as your budget i'm sure you will fall in love with something!