Steam key activation

[pacmanlite]

I know this has probably been asked a billion times before but once I have got my steam key how do I use it on steam to get the game? Where do I input the key?
Thx Pacmanlite c(｡◕‿◕｡c)

[Klonan]

[pacmanlite]

Thx Klonan
It turned out that the problem was that I had some how copied my upgrade key into the bar by accident <DERP>