Themes?

[Aluenda]

Hi, I'm like totally new to php and Rusnak, and some help would be greatly appreciated!

My site is http://capria.nuvarta.com/index.php, and the person who set up the host also installed Rusnak php onto the site.

I'm interested in changing the theme, or at least learning how to, because I want my site to look a bit more professional. I also have a banner being made and I want to know how to insert that into the theme :)

If anyone can help me, please do! :D
Also note that I know next to nothing about php (I do know html very well! and a little bit of css ;D), as this was installed for me, and I would like to avoid downloading things straight to my PC (my other computer crashed because I had downloaded something into it :o)