How can i get rid of seborrheic dermatitis?

Possible food allerg: Could be a food sensitvity and/or deficiency of omega-3 fatty acids (usually supplemented by taking fish oils). Try eliminating foods such as grains and dairy for a couple of weeks. These are the most common culprits.

Look for a shampoo..: ...With either tar, salicylic acid, zinc pyrithione, or selenium sulfide (or a combination of them). If none of those are strong enough you'll have to visit a dermatologist to get a prescription-strength shampoo, plus perhaps a steroid foam or lotion.