Well You have a great starting budget, and unlike many other people who stumble their way onto these humble forums, you actually did some research first it seems.
First thing many will tell you is that you need to get age verified. Becuase unless you do, no one on these forums will tell you where to buy. If you are not 18 yet, well then you have a lot of time to do more research and find out exactly what you want.
You are asking about your first gun, what kind are you looking for? An smg or a rifle? CQb or outdoors? Bullpup or standard.
In your price range there are tons of great options out the for you CA and ARES g36s fit in there, and they are beautifully built, durable, and pretty reliable. G&G has most of their stuff (other than the L85 or the FN2000) in that price range. You are actually coming into it with a much larger budget than most people do.
If you get age verified you have even more options in the classifieds, and usually, you can get some extras in with whatever you buy, be it batteries, chargers, optics, spare magazines, different grips, stocks , rail covers etc. You can find on any given day for sale in there a billion (an exaggeration.. but not my much) m4/m16 variants, g36, SCAR, p90s, AK's, mp5, you get the idea.
Personally, if you are going brand new, id say the ARES g36, its got proper Hk trades on it, its fairly lightweight, comfortable to shoulder, you dont have to worry about body kits or a ton of aftermarket parts, the iron sights are decent, the body is not some cheap garbage plastic. It has the quick swap spring, and even after taxes and shipping, you would have a little bit left over to put into spare mags, optics, or even a couple minor internal upgrades from your local gundoc. All fitting into that 550$ budget of yours.