Problem with CJ20N transaction

Hi, i want to add a search help or match code in the Project System User Fields on standard transaction CJ20N

PRPS-USR00

PRPS-USR01

PRPS-USR02

This matchcode will be in the standard transaction CJ20N

Someone tells me I should include this logic in a user exit. Can anyone give me some ideas , how I can accomplish this task?

A step by step procedure, please

I´m beginner in ABAP, and I don´t know how to do a user-exit.

Reward Points

Thanks a lot.

José Angel