m,
First, add a new column "before" the existing column A. It's not possible to combine the current content of A with the current content of B and have them appear in the place of the original Column A data. So, we add a new column for the combination. If you don't want to continue to display the original First Name data hide that column.
When you combine two character strings, it's called Concatenation. There's a concatenation operator and a concatenation function. You can use either. Here are examples that meet your spec:
In the first data row of the new column A, write: =C&", "&B
Copy and Paste or Fill to the rest of column A.
Regards,
Jerry