Ok, here ya go:
Let's say in A1 it says "FIRST", and in B1 it says "LAST".
If you want cell C1 to say "FIRST LAST", type in the following: =(A1&" "&B1)
If you want cell C1 to say "FIRST, LAST", type in the following: =(A1&", "&B1)
If you want cell C1 to say "LAST, FIRST", type in the following: =(B1&", "&A1)
Apply it to whatever cells you're using in your case.
ETA: Oops, Dan beat me to it.