When you're working with software of any sorts, it's always wise to read up on documentations:

http://learn.elgg.org/en/1.10/index.html

If you're gonna change the looks and feel of your site, you'd need to develop a plugin so you can make it the way you want it to look. The way Elgg works is through overriding views with php files and registering functions in your custom plugin. You would need to learn these:

• HTML - fundamentals for a basic website
• CSS - layouts, colors, etc., mainly for aesthetics
• PHP - logics and functionalities
• Javascript - Not required but it could enhance user experience