new to wordpress

this sounds like a very stupid question and probably is. what files in wordpress do you directly edit to affect your page? i’d like to make a simple horizontal menu bar running along the top of the page, preceded by some banner graphics or what not. thats pretty much it. and maybe attach a background to the page. my requirements arent really all that taxing but im beyond new, prenatal in fact, to this and would be extremely grateful for any and all help provided. thanks so much and have a nice evening.