CS: GO Help please?

Okai dudes, i've just finished downloading CS: GO from Steam and every time I launch the game, It just goes to a black screen. I thought closing it and opening it would help but once I pressed Alt+Tab (to get to the desktop), a popup appeared saying "This program needs Steam" which confused me quite a bit since I opened it from Steam... Any help would be greatly appreciated. :D

As with all Steam issues have you verified local files. It sort of the Steam version of "have you switched it off and on again"

 

Right click on the game name in the list on the left. Chose Properties. Go to the "Local Files" tab and click on "Verify integrity of game cache"

 

See if that sorts it. If not then specific trouble shooting can begin.