To change the look of your battle menu, you'll have to edit the graphics you can find in your "Pictures" folder. The ones you're looking for will have "battle" as a prefix before the rest of their name.
As for making it appear horizontally? That requires some editing in the scripts, which can be pretty tricky; especially if you don't know what you're doing. You can find a tutorial by Maruno on the wiki - just look up "battle command" and it should be the first thing on the list (I'd give you a link but I haven't posted enough)- but if you want, I'm willing to see if I can fix it up for you.