Welcome to Android Central! Are these ads being shown within certain free apps, like as a banner at the top or bottom? In these cases, you can typically get rid of apps by paying the $1-2 (or equivalent, wherever you live) for the full ad-free version. Remember that those ads help to support the developer and allows him or her to make more apps.
If you're getting ads displayed as popups on your homescreen or as notifications in the Notification Bar, then you most likely installed an app that includes adware. Try installing something like Lookout's
Ad-Network Scanner & Detector.
