Hey & welcome to GT! If you haven't, I encourage you to work your way through the Fundamental Courses. That's the best place for beginners to start.

Originally Posted by: Deanhorneck

I am trying to learn the C chord and am wondering where I should put my thumb on the back of the neck.

[p]Your thumb should be placed directly opposite your fingers on the back of the neck. Your thumb should provide just enough pressure to keep your hand firmly in place on the neck and opposing pressure to help your fingers hold down any fretted notes.

Sometimes beginners squeeze too hard with their thumb and, or their fingers. Actually, this is sometimes a problem for some skilled guitarists, too! Remember that you want just enough pressure to get the job done. Not enough pressure will result in a poor sounding chord. But too much pressure is wasted effort and can over time hinder your ability to move quickly and effectively to the next chord or hand position.

I demonstrate this in the 3rd lesson of this tutorial:

https://www.guitartricks.com/tutorial.php?input=605

Hope this helps! Please ask more if necessary. Keep practicing!