automatical testing tool for android

I have been developing an app on my own. Gradually I found it`s really hard for me to test the app all by myself. I need some automatic testing tools (or framework) for both unit testing and ui testing.

Unit testing supposed to be like the common testing tool, but UI testing tool are better to support multi device with all kinds of screen sizes, os version and SD cards.

If there are some tutorial introducing how to use it step by step will be greatly appreciated because I am total new with testing Android.

Thanks in advance.