2000 S10

Hey all,
I have an S-10 with the 4.3L V-6. My "Service Engine Soon" light came on. It is not running rough. It starts right up and runs fine. What usually causes this to happen? I am not a mechanic, so go easy on me.
Thank you,
John Cornell

a lot of things can set off the light from a loose gas cap to a bad component, really need to get it scanned to find out code that is set