New to WP Need Help!

Okay, I understand html and css, but I am not used to working with other people’s code- especially when I can’t control it from the server side? I don’t understand how I am supposed to upload images. I thought that the template would take a more “facebook approach”, but apparently you have to go edit the css and place files into your server? I am just confused about how it’s all supposed to go together, find the files needed, and not revert to what is already live. I have been trying to make my header appear instead of the template picture with no success. Please let me know step by step what I need to be doing. And yes, I know there is Codex support and the Wiki on all kinds of this stuff- but reading technical jargon is very time consuming and it generally doesn’t get me where I need to go. So please- only link to it if you think a relatively new web-coder could fully understand it. Thanks!