I'm not 18+ (17 go figure), but I might be able to help!
Anyway, the biggest piece of advice I can give you is to keep taking practice tests. Look at the reasoning for why each answer is right or wrong and get a feel for how the test is created. If you don't feel like sitting and taking a full test (which I wouldn't do either with my attention span, haha), just take one section from time to time.
For the writing sections, if you can find any examples of good and bad essays, then read them and look for explanations for why an essay scored the way it did. You could use this to learn what to do and what to avoid doing. Since it's much more difficult to self-grade an essay, you should ask someone to read over your essays and grade them for you if possible.
If you have trouble with time constraints, then start by taking the test untimed and see how long it takes you to do it. After that, start shaving off a couple of minutes until you can comfortably complete the test within the time limit (or even a couple of minutes before that if possible).
Oh, and here's some tips for you to do on test day-
Sleep well the night before and eat a solid breakfast that makes you full but not too full (or else you'll feel sleepy). I usually drink a bit of coffee before a test as well, but if you never drink caffeinated drinks/don't tolerate them well, then I'd suggest you skip that. Try reviewing a bit before you take the test too, not so much to cram things in last minute but to get your mind jogging before you start the test.
Lastly, I'm going to agree with Xiz and say you should relax. You'll be fine, really. Even if you don't feel that way, just know that stressing out will only hurt you. It sucks quite a bit to have nerves during an exam, so just do your best!