That is a difficult question to answer without seeing the rash. Basically, you need to see a physician.
It could be caused by a number of things- commonly perhaps an allergic dermatitis, eczema or scabies (an infection). These can all be treated. Itchy rashes are often helped by moisturising creams- E45, dermolol etc. but you may require specific treatment (e.g. for scabies) so you should see your doctor before self-treating.