someone help me!

I am trying to figure out how to add layouts to my blog! I have a new header with text BUT it isn't as big as it should be! I have tried to follow online directions to change the background but so far all I have done is center my blog (which is great because I wanted to do that too!). I really need some help from someone who has done this before! Why won't the direct link I'm putting in the html work for me??