If you want to diagnose the code yourself... this is how you do it. It is very simple, just follow the instructions.
How to Read Malfunction Codes for Your DOHC 240SX
If this is too complicated of a procedure for you, just take it to autozone. They will read the code for you and tell you what is wrong with the car (FOR FREE). My guess would be IACV-ACC Valve malfunction aka code: 25 since this is a very common problem with ka's. Does your idle jump around?
P.S. Better the check engine light than the "change engine light"