From your email, it sounds like you received the serial key from Humble Bundle.
So from there, you must install the Steam client onto your computer and have an account with Steam (if you haven’t already).
Next, start the Steam client (NOT the website) and enter your login and password. At the bottom left corner of your screen it should have “+ ADD A GAME…” Click this and follow the prompts to enter the key you received from Humble Bundle.
If you don’t see “+ ADD A GAME…” at the bottom, make sure that “LIBRARY” is highlighted at the top of your screen. If not, click “Library” and then look at the bottom right.