Realize Nothing is OriginalStop trying to come up with things from scratch, even the world's best artists have to start somewhere. Once you can get past this you'll have a much easier time creating things.
Let Yourself Be Inspired
This is really the key to becoming more creative. People who think they're not creative just aren't letting themselves be inspired enough. If you don't know where to start, get inspired by other people's art: DeviantArt, Tumblr, Pinterest and Flikr are all good places to start. Try letting something completely random from the real world inspire you-- converse sneakers, kitchen tools, insects, taco bell commercials, etc.
Take Your Time
Don't try to rush creativity. Wait until you really have some time (preferably alone) to really go at it.
Don't Be Afraid of Screwing Up
I can pretty much guarantee that you won't like the first thing you create; whether it be written, sung, composed, painted, decoupaged, sketched or whatever else. But that's okay, if you're just being creative for fun, there's no reason you need to be hard on yourself. Throw it out and start again. I can guarantee that any successful artist didn't create something inspiring their first try.
There's a reason "practice makes perfect" is such an overused phrase-- it's true! You can't expect to get better if you try something once and quit. So your first effort at drawing a Day of the Dead skull looked like a flower patch vomited on the page. If it's something that you're really interested in, you'll get better as long as you keep trying.