If you're just practicing for now you should just pretend that someone's asked you to make a website for them, something simple. The other week I made one up for a made-up art gallery, do something along those lines. A simple or small business, maybe a clothes shop for teenagers or something.
Before you get to programming and content production, throw around some ideas in your head, or just write them down. Brainstorm, what things do you think of when you think of the thing you want the site to promote? Say if it were for a teen clothes shop, I'd think of surfers, skaters, rock music, graffiti and that kind of thing. Then start on your mockup, using your brainstorm to guide your artistic direction.
Then, once you're happy with that, you're ready to start producing your site :D