Presumably you want some hints, rather than the answer (learning is not as effective otherwise)? So:
1 - functions require square brackets, even if there are no arguments
2 - some functions require arguments
3 - Plot plots a function as a function of a single variable (not two)
4 - variables supplied as arguments to a 'function' are bound using patterns with underscore characters on the Left Hand Side
5 - the multiplication sign can be replaced with a space between two operands
6 - Plot3D plots a function as a function of two variables
