Can someone give me the correct syntax for building a Lookup () formula.

The user will give me sheet name (i.e. Sheet1) and I want to use that value
to build the formula. What is the correct way to use Sheet1 to build the
formula. Specifically, where do the quotes and double quotes go to build teh
User Entered Value for teh Sheet and Range Name?

=Vlookup(23, User Entered Value Here for worksheet and Range Name, 3)