SES light coming on. P0128 code from auto parts place. what do i do to fix this myself?

It depends on the vehicle. 9 times out of 10 this code means the thermostat has failed, of course I'd try that first. I do not know what mechanical skills you possess or what type of vehicle you're asking , so I can't really answer the second part of the question. Some tstas are easy, other ones require a lot of work.

Check throttle body motor

CHECK ALTERNATOR VOLTAGE OUTPUT.I HAVE SEEN WHERE ALTERNATOR VOLTAGE REGULATOR GO BAD CAUSE BATTERY VOLTAGE DROP TO 11.5 VOLTS. WHICH WILL CAUSE TRAC LIGHT AND CHECK ENGINE LIGHT TO COME ON. BUT HAVE VECHICLE SCANNED AT AUTO ZONE OR ANY AUTO PARTS STORES THEY WILL SCAN FOR FREE AND TELL YOU YOUR TROUBLE CODE.

See or call your local auto parts store such as AutoZone or O'Reillys for the trouble code definition. Also, make sure your gas cap is really secure, and I mean tight.
Rick

2001 accent gl 1.6, p1123 code comes on. how can I repair it myself?

after you replaced the gas cap did you clear the code
if not the SES will stay on showing the same code in HISTORY
if you did clear the code and the SES light is on and the same code is CURRENT
then you could need a EVAP SOLENOID
you may want to get a SMOKE TEST DONE on the EVAP SYSTEM
hope this helps

Replacing it is the only fix. Sounds like it may be sticking open slightly.

replace the thermostat.

first problem, went to autozone. the guy that looked at your car was making pizzas last week. they pull codes and sell you every part you will buy. take the car to the saturn dealer and replace the bcm.

How To Diagnose P0128 Faulty Thermostat