Can't seem to download from the app center

Hi, I'm completely new to linux and I'm having some issues installing apps.
When I go to the app center, find an app and click 'install' it comes up with a loading bar which says 'starting' but never moves. So far I've tried to install steam and Firefox but both with the same result.
If anyone knows how to fix it, that would be fantastic.

Thanks

 

Try update the system first, open the terminal and type:

Code (Text):

sudo apt update
sudo apt upgrade