what do i do when my microwave starts sparking?

Remove the cover and clean all the baked on grease from it and the area where it is attached. That should take care of the problem.

EDIT:

It would be hard to count the number of microwaves I have repaired where this is the problem. Answers saying to throw it away and buy a new one is silly and provided by the unknowing. If people would read the instruction manual it will tell you to keep the waveguide cover clean. Grease will accumlate around this cover and after being subjected to microwave energy it turns to carbon, thus causing the arcing. Just because you disagree with my answer does not make it incorrect. Give my solution a try first and save yourself some money.

Are you putting any metal object into the microwave? That will cause sparking. Never put metal into the microwave.

Replace the microwave! You will spend as much as a new one or more to get it repaired.

Sorry to be negative, but as the other answerer said, it could be metal you have introduced into the microwave chamber.

It could be a blown seal, if so DO NOT use it, you'll be putting yourself at risk of radiation burns internally.

If I were you I would dump it, why take the risk?

have a good appliance man look it over or replace it with a new one. might cost about the same either wat depending on the model