I suppose 18 is an adult. I don't think it's practical for the OP to go about it this way though i.e. "just don't blunder." I certainly don't think he'll be 1800 in "an year" or by his "third tournament."
It would be interesting though if he did intensive tactics training a la De La Mesa and reported back to us.

Yeah, if you play when you're a kid, you don't need to know much. It's just a little sickening to see analysis of, say, 2300 teens who dismiss good moves or glaze over bad moves because they (apparently) only got there by good calculation / tactics / endurance.
Unfortunately, if you're an improving adult, you can't take the path of "just don't blunder."
I was 18 when I started playing (and 19 when I played in my first tournament). I disagree. Not hanging stuff (well, by which I mean to obvious tactics. Deep/tricky ones sometimes can't be avoided) and knowing the openings that you play decently will get you to to at least 1700.