I did not realise before that not everyone knew how to redeem steam codes so heres my quick tutorial also its the same as redeeming as a steam wallet code.

Step 1. Look at the bottem-left of steam look for "+ ADD A GAME..." and click it
Step 2. A menu should popup and click "Activate a Product on Steam"

Step 3. On the new box that pops simply follow its instructions (or skip it by pressing next) till it asks you for a code, then enter your code and done

Step 4. PROFFFTTT!!!!!! (mlg meme was mlg)