Web Design Really Need Help With Changing This!!!

Hello everyone, thanks for reading this and your help is greatly appreciated. My question, I downloaded a template of a Ski Thing and had hopes of changing that. But that is the problem, I dont know how. From reading through some other threads I think it is something to do with the .css, would I be correct??? Anyways do I have to make up my own "stylesheet" and save it as .css to put innto the html code??? Thats all, and as I said before all your help is..needed but greatly thanked, Jkc123