dog constantly itches, irritated skin in armpits

well the title says it all.. she is always itching.. also she has red sometimes scared skin behind her ears but mostly underneath her armpits.. i have tried giving her fish oil but i was advised thats not a wise thing to do. also tried putting diatomaceous earth in her water but yet again advised it was an unwise thing to do from my parents. what can i do for her? if not, what could a vet do for her? Tweet