I changed the fuel filter and oil and now the service light is coming on. What could be the problem?

 

so you don't play the guessing game...

buy or borrow a OBD II code reader/scanner from autozone/checker/etc...

this was my first "MOD", so I know what's going on before I bring my truck into the dealer...

 

None of what you did should bring on a SES light??? Well , maybe one thing; are you sure the fuel filter is flowing in the right direction? They can be put in backwards and then you wouldn't have enough fuel going to the injectors.

Btw, Auto Zone(and others?) will pull the code(s) for free.

Dan