Are you able to create an app composed of just HTML, CSS, and JS? If so, you can build a "web app," which is a mini website that was designed for small screens (rather than just the desktop screen), and people navigate to it through their mobile browser.
Or you can build a "hybrid app," in which you use your skills to build the app, then use PhoneGap or other tool to use the phone's features, such as camera and geolocation.
It won't be easy. For instance, if you go the hybrid way to make iPhone apps to sell on the App Store, you'll need to download and learn PhoneGap, download and learn Xcode, and understand how the App Store upload process works.
My sig gives several articles on step-by-step instructions to creating and uploading apps to Google Play and iTunes App Store.