I'm taking antibiotics that upset my stomach. How do I know what foods can I eat?

It depends. In general, it depends on which antibiotic. There should be instructions with the antibiotic as it was prescribed to you--usually something printed by the pharmacy--that go into this in some detail. Also your pharmacist can tell you. But it really depends on the drug: tetracyclines require avoiding calcium-containing products. Linezolid requires avoiding wine and certain cheeses.