You use the function CONCATENATE or CONCAT or the symbol & as follows, assuming last name is in column A, first name in Column B
=CONCATENATE(A2,", ",B2) will give you Aperals, Paul
=A2&", "&B2 will give the same result
For what it's worth, you're better off with two columns as your basic data base, because it gives you a lot more flexibility in the long run. With two columns you can assemble the output as "Last Name, First Name" or "First Name Last Name"; you can also easily sort by last name, which is much more difficult if you stored the data as FULL NAME ONLY.