You do that stuff by editing the skin that displays your blog. Here's the manual page that you'll want to start with:

[url=http://manual.b2evolution.net/Edit_evoskins]Edit evoskins[/url]

I have no idea how to do that.

Ok, I can walk you through the changes. First, open up /skins/[the skin you're using]/_main.php in a text editor. You'll probably need to download it with your ftp client first. Once you've made your changes, save it and upload it back to the server. Skim through the _main.php file and you should see lots of comments. Those are the lines that begin with "//". Read the comments and they'll tell you what each bit of code does. Find the sidebar section, then find the bits in that section that you want to remove and delete them.

Okay, I made a load of skins. Thing is, when I scroll down, the background like dissapears, as it doesn't continue forever...
So... where would I stick a "fixed" tag?

I don't have a very good imagination. Can you give a link to your site so I can see this problem in action?

Oh, and by "make", I mean't copy one skin and change the background for mines XD

here's the actual page:
http://hollowshrine.uni.cc/blogs/users/mogster/index.php

scroll down to see what I mean...

now which part do I put a fixed tag in?

add this to your body {} style in clelbleu.css:

BODY {
. . .
font-size: 84%;
background-attachment: fixed;
}

*huggles you*

ps. teh like hugs ;p