I have a 98 civic and I'm going to replace the timing belt on it, what else should I replace when doing the timing belt. I'm about to it 100k miles, is there anything else I should do.

any and all other belts, water pump, thermostat, plugs, plugwires, rotor and cap...basic tune up stuff that hasn't been replaced recently.

Replace leaking cam seals & crank seals if needed. Replace water pump/coolant as already suggested. a few others : Valve cover gasket & seals, valve adjustment, drive belts(alt/ps/ac)

Engine light is on. Already change oil,and filter, spark plugs and wires on 00civic si vtech. what else could it be

Engine light is on. Already change oil,and filter, spark plugs and wires on 00civic si vtech. what else could it be
It could be many different things, the only way to find out is to retrieve the error code. Check engine lights don't come on from maintenance items.