CSS Help with Menu

I am fairly new to WordPress and I know enough to make myself dangerous 😉 On the site that I have created through WP, I have a menu, and it is vertical, my boss wants it to be Horizontal. I have read through the various help topics, searched through the forums, and while I have found many things that have helped with other issues, I have not found a solution for this. Can someone help me? I don’t know where to begin, let alone what I am doing wrong. I have never dealt with CSS before now, if it helps to know that. If there is some information I can provide for someone to better help me, let me know and I will do what I can. Thank you in advance!