How to reset the check engine light on a 1999 toyota avalon xls

You need to have a code reader/scan tool hooked up to retrieve the code/s that triggered the engine light. You can have this done at most auto parts stores for free. Try Auto zone, Checkers, Napa, or Murray's. The engine light will eventually go out if the computer does not receive any codes after many cycles turning the ignition on and off. It will not go out if there are codes still present. After you've retrieve the code/s bring them back here and we will have a better understanding of what caused them.