Customizing my blog

I don't have the SLIGHTEST clue about html....and I think that is what I need to know for my blog. I wanted to design my own header...but if someone knows what I need to do so that the header goes all they way across the top.....such as dimensions/size, html code.....I would greatly appreciate the help. You can e-mail me if you don't want to give away all your secrets! LOL