Which words do I pick
Grammar? forget it.
I once had a writing experience that was truly sublime. The content was a series of stream-of-conciousness, on-the-fly anecdotes that I scratched out on a phone. It was completely raw but thoroughly awesome. This can't be good advice but: try not to edit too much.
A piece can take on different tones. Conversational, eloquent, formally minimalist. It'd be nice if this site picked one and stuck to it, but that constraint is a rather harsh one. But let's say you're a balding 40 something trying to crash a college party and you need to send a text regarding the whereabouts of the shindig. Well, you best come across as casual lest the host peg you a narc. Imagine you sit down, write some words, and something terribly formal comes out.
Delete that garbagio. Begin your first sentence with "okay, so..." and the next words that come to you should be different this time around. That oughta help loosen your tie. Don't worry, you'll get a chance to use all the big words you know when you crash next days' lecture.