autoamtic increment in primary key field

I have on ztable.

I have created mandt field and

My plan is to create one field as PRIMARY KEY which increments automatically when ever user create a new record .

for example:

MANDT MANDT PRIMARY KEY

COUNT I (which one can I use) PRIMARY KEY

plant WERKS_D

mat type MTART

mat grp MATKL

CREATED_BY SYUNAME

CHANGED_BY SYUNAME

Where to write code to get count get incremented automatically for the new record which user is creating ?

Please help me sooner or later.

YOUR HELP IS HIGHLY APPRECEATED.

Edited by: Rob Burbank on May 1, 2009 2:45 PM