to customize themes you don’t need to know much PHP. All you need to know about it you can find in the WordPress Codex. You do need basic knowledge of (X)HTML to edit the content and be pretty familiar with CSS to change the design of the blog/site.
If you want to learn (X)HTML and CSS there is a great site called HTML Dog which has tutorials that teach you everything you need to know. It also teaches you to do things the “right” way instead of the “looks alright” way like 90% of the websites do.