New to Tumblr

So, my title says that I'm new to Tumblr, but that's a bit false. I only say I'm new because the Tumblr I did have was deleted and I decided to start fresh. That being said, I was wondering if anyone knows how to make a Tumblr theme that is like customized. I don't know the first thing about customizing Tumblr, so help would be much appreciated. If you need any kind information on what I'd like the customization to be please feel free to ask any questions because I will answer what I can.

On another note: I do have a contest up and running. It is called Avenged Sevenfold Countryfied. There are still spots available if you are interested in joining. The deadline to enter is coming up pretty quickly. The link is posted below:

Avenged Sevenfold