i don't write first-person very often, but when i do, i focus more on thoughts and feelings really. i don't describe things much, i used to be super bad at that, but now i'm better.
it usually depends on what i'm writing, what style or feeling it needs. is it comedic? is it dark? romantic? usually if it's romantic, i'll have a good number of run-on sentences. if it's comedic, i try to make it as funny as possible (with a bunch of dry statements and witty stuff) without making it crack.
all-in-all, i pretty much adapt to whatever i'm writing and just make sure i get into that frame of mind before writing it.