Help with Homepage

Hi, I am totally new to WP and I'm undergoing quite an intense course of 'self tuition'! Anyway, have done reasonably okay but I'm struggling with a couple of things; firstly, I cannot change the graphic on the homepage? I've tried everything I can think of but without success. Secondly is there a way that I can reduce the height of the header? It has my logo in it but is too big and I would like to slightly reduce it. Finally, how do I change font of the titles in the menus (homepage). Sorry for the basic questions as I'm sure the answers are quite obvious. Many thanks,