Left hand thumb?

You shouldn't necessarily press your thumb into the back of the neck. It should be there, but just enough pressure to keep your hand stable so you can press the strings down enough to produce clear notes. Quite often beginners will overcompensate for these things and it can lead to hand health issues down the track. Your thumb position will changes dependant on what you're playing at the time, so adopt correct technique but it should be comfortable too.

 

Look at the way you naturally grab a glass of water. Gently curve the fingers, with the thumb somewhere around your 1st and 2nd finger. Now put a bass where the glass of water was. Voila!