You can't prove it- that is not true. And it probably was not what you were asked to prove. What is true is that any norm gives a norm. That is, given any inner product, we can use it to define a norm- but the norm and the inner product are obviously NOT the same.
Given any inner product, u*v, say, we can define the norm of a vector v as |v*v| where "| |" is the absolute value as defined on the field F. You are asked to show that |v*v| has the defining properties of a norm.