as said above, karma isn't instant and noticeable, nor will it be a big grandiose thing. say you hold a door open for someone, don't expect to find 20$ right after and a banner to fall down from Karma saying 'you're welcome'. say you hold the door open for someone, at the end of the day someone might hold the door open for you. or, if you always hold the door open for people, this could build up and [as an example] one day that week you go to the vending machine for a snack and two fall out instead of one.
same with negative karma, you could trip someone intentionally, then loose your favourite pen, or have a pop quiz later that day in a class you don't like. little things, karma doesn't tend to do big things that obviously scream karma if you're just doing small kind gestures. and as said above, don't go around doing good expecting karma to rain money on you.