If you have an error like this,

1. right click on project name, open properties.
2. Select Java Build Path (from left menus)
3. Under the "Libraries" tab, find the entry 'Unable to get system library for project', select it then click on the Remove button.
4. Select Android (from left menus), tick on the version of android you want to target.
5. Select Project menu (main top level menu), select Clean... Select OK.
6. Right click on project name again, go down to Android Tools > Fix Project Properties.