Here is the full list of all 17 Just Sign! achievements.
Finish tutorial
Finish first world
Get gold in all garden levels
Finish second world
Get gold in all underground levels
Finish third world
Get gold in all city levels
Finish fourth world
Get gold in all city in skies levels
Finish fifth world
Get gold in all sea levels
Finish sixth world
Get gold in all space levels
Get gold in all levels
Build 50 turrets
Kill 500 enemies