damsel in distress with electrical problems

I sadly have an 01 tracker. The battery is good and the alternator seems to be ok sometimes. the battery light keeps coming on after the engine gets up to operating speed and if u check the alternator then it isn't putting out anything, but on a cold engine it is fine. i don't want to replace the alternator if i don't have to since everything on this truck is expensive. any ideas?

check to make sure your cables are tight, check for corrosion, when your engine is warmed up, what is the voltage output ? do you have a voltmeter ?