GTN,
As a fellow resident of WA and a gun owner here for, hm, probably a couple decades, our local gun culture is actually quite strong (yes, in spite of recent initiatives). City dwellers will be much less familiar with firearms than suburban and rural folks, though. I don't hide my interest but I also don't broadcast it and I do keep in mind the company I'm with.
There is actually quite a lot available to us (for now).
Copule issues:
Hand Guns
These you won't be able to legally acquire in WA until you're 21 years of age. However, they'll tend to be the more affordable firearms.
Rifles / Long Guns
As you know, you can legally buy these at age 18. However, the HK long guns are expensive. To the point that as someone with a household income in the (very) low six-figures I still don't see the value in them. They are awesome but I can't bring myself to pony up the money for genuine HK guns. That said, I am appreciating the HK Clone market. They are still expensive but more accessible and based on the HK design. I might suggest looking there as others have suggested also.
I-594 / Purchasing
As much as I want to puke thinking about I-594, I have conducted some sales under the new rules. It was enough of a PITA that under other circumstances I'd just assume buy new as it's not worth the hassle for the price discount of a used gun. However, I was the seller trying to raise cash for my own new rifle purchases and went where the buyers were (some local, some out of state). It was still a PITA of coordinating a sale with a buyer, and then finding a gun shop that was open and mutually convenient to go do the in-store background check. But we got through it.
Otherwise, all of my purchases have been new firearms to date and I-594 didn't impact those in any way. Even my on-line purchases (which are required to be shipped direct to my FFL) weren't impacted by I-594. I wish it wasn't voted on but it hasn't slowed me down.
Specific Suggestions
This will depend entirely on your interest but I would factor ammo costs into the equation, too. I recently picked up a PTR91 (HK 91 clone) and as much as I love it I went into it knowing I'll be shooting it less often than other guns due to the cost of .308 ammunition. But the rifle was ~$1k, magazines are ~$5 ea, and I really like the design. It's a relatively cheap(er) HK patterned rifle. So . . . I bought on. That said, YOU need to determine what YOU want. Figure out the intersection between your interest and your budget and then start saving up.
HK MP5-22
I have one and it's one damn fun plinker! Cheap to purchase ($350-ish on Armslist all day long or ~$475 new). Cheap to shoot (.22lr is till cheap vs. other ammo). Gives you the form factor and feel of the MP5 without costing you 4x as much or more. Hard not to recommend this to start with. Plus, you can have a lot of fun with this while saving up (which may be a while), then sell this to put you over the top on a different rifle.
Century C93 (.223)
It's like an MP5 but in .223. The magazines are a little expensive and the .223 cartridges are more than 9mm but. Damn, hard to beat the price.
Vector V53 / V93 (.223)
A nicer rifle than the C93 but otherwise "the same thing", a .223 MP5. Cost is more but still much less than a 9mm MP5.
PTR91 (.308)
Solid rifle, well built, amazingly affordable for a ".308 battle rifle." Mags are cheap and plentiful but the ammo is expensive.
Try Before You Buy
This is going to be rough right now but if you can, try before you buy. You might find you don't really like how a gun shoots or handles. And it would be good to find that out before dropping serious coin. If you develop a social circle of fellow gun owners you can try each other's stuff out when you go shooting. I have a few friends I do this with. Also, ranges sometimes have rental guns you can try out. They most likely won't have any HK long guns but they might have some of the newer pistol offerings. Point being, it's good to have experience with a wide variety of guns to make an informed decision on. And I do recognize this will be tough with your funding situation. Rental fees, range fees, and ammo costs add up fast.
Oh, and the reason everyone is giving you grief is . . . . . we've all been in your shoes before.
Take a breath, slow down, focus and tackle this one step at a time. Some patients AND determination will go a long way and you're going to need both. As you hit 18 and get out into the adult world you'll be surprised how much of your income is eaten up with necessities. But if you work hard you can make a go at this. If you're determined, a second job can help finance some extra toys. After all, what else are you going to do in the evenings watch TV? Surf the internet? Or make some money?
Best of luck.