I have a heat rash on my right hand and was wondering if antihistamines would help or if not what is the best way to treat it?

Probably not: This appears to be a contact allergy or irritant reaction and as such an antihistamine would not likely do anything. Avoidance of the contact agent and a topical cortisone cream may help. In the future, attaching a photo of the rash may help us answer your question more accurately.

Unknown rash on hand: Not sure what you mean by a heat rash. Other than hives and urticaria i don't know of a rash that will benefit much from antihistamines. You may need to get someone to take a look and make a diagnosis to choose an effective therapy. Make an appointment with a physician if it is chronic or recurrent. If it is mild consider a topical Hydrocortisone cream otc.