Rashes may be due to allergies,viral/bacterial/fungal infections, bugs,exposure to sun,acne,eczema, stress etc..
-Wash your arm daily with mild soap and warm water.
-Use hydrocortizone creams for allergic reactions.
-Avoid scratching the arm, as it will spreads very easily.
Avoid jewelry that causes rashes.
-Calamine lotions can be used to relieve from itching and irritation.
-Applying ice wrapped in towel/cold compresses in the affected area helps to relieve from itching and irritation.
-Applying aloe vera gel/olive oil/chamomile oil is good to relieve from this.This a also a good home remedy.
If it persists after this then consult a skin specialist.

Hello,
For arm rash good treatment is using anti rash cream or apply Aloe Vera pulp on it...

Skin care is totally depend on you . I 'm prefer do  some tips for my healthy and shining skin :-
1. From the sun protected yourself .
2 . don't smoke .
3. Eat more vitamins and supplements foods like tomatoes , potatoes and
    green leafy vegetables . But avoid healthy foods .
4.  Drink plenty of water .
5.  Do some exercise , yoga and medication .