Proving that a BST with N>=1 nodes will have log(N+1) levels

Question

I am trying to prove by induction the following theorem: Use Induction to prove the following fact: for every integer, $N\ge 1$ , a BST with $N$ nodes must have at least $\log( N + 1)$ levels. I've proved the base case but I am struggling to figure out how to apply induction to prove for the $K+1$ case. Any suggestions would be wonderful.