how to take first letter of 1 column and the whole word of another columns

Hi

please advice or give an example i want to a table with columns  First_name and Last_name and i want to create insert it in a third different column DB_USER

example

Table  - columns   First_name  Last_name    

                             Jesus           Gutierrez

and the function or code insert it into a third column

                             DB_USER

                             JGUTIERREZ  

Please advice

Jesus