web app? android app? ios? windows 8? blackberry app? facebook app?
I'll assume that you need an android/ ios app for now, yes you can use an online generator
http://www.apps-builder.com/
http://appery.io/
http://mobile.conduit.com/
http://www.theappbuilder.com/
http://www.appypie.com/
read this
article too. A small search for "online mobile app builder" or "online ios app builder" will show you many sites, there are app builders out there than the entire chinese population and that's because you can create apps using html5, css3 and javascript, you won't be creating native apps, which has a plus side and a down side, you don't need to worry about it.
If you know html5, javascript and css3, you won't ever need those builders either, you can write it from scratch.