Defining variables inside a function Haskell

I am a huge newbie to Haskell, I actually just started 10 minutes ago. I am trying to figure out how to define a variable inside a function. Lets say I have the function

foo :: Int -> Int
foo a = 
    b = a * 2
    b
-- Yes, I know, it doesn't do anything interesting

When I run it in GHCi I get a syntax error! How can you define a variable inside a function?