If a key is needed for your game, there are 2 ways for you to find it:
1- When you first launch the game, Steam will automatically display any key needed for further activation.
Please note that this happens mostly for older games.
2- In your Steam Library, click on the game in the list of games, then, click on the cogwheel icon
on the right, followed by Manage
and then on CD Keys.
Some games do not require key activation, in this case, you will not see the CD key
option. Also, please note that the CD key
option will only appear if the game is already installed or if the installation is started.