Temples are holy places and the sanctum sanctorum is the holiest place in the temple. Sanctum Sanctorum is a Latin phrase translating the biblical term meaning holy of holies. The inner chambers of Indian temples where the main deities reside are also considered the holiest places but every inch of area right from the entrance of the temple is considered holy by Hindus.
Temples are constructed according to several Agamas or temple construction rules to imbibe them with divine power within the confining walls of the temple. Hence, there are several do's and don'ts when you enter a temple.
All visitors to temples should leave their footwear outside the temple, since footwear is strictly prohibited inside temples. Usually, there will be a place or multiple places that allow you to leave your footwear both free of charge or for a small payment and issue you a token.
You could get back your footwear by returning the token when you come out of the temple. However, in many temples, the open corridors can be quite hot during sunny days and you should be ready to tread carefully on such floors.
In all temples, you should use only your right hand to hand over donations or receive holy water and other prasadam items that the priest offers. Left hand is for use in toilet and considered dirty. You should also conduct circumambulation of the temple and its various chambers only in clock-wise direction.
Your dress also should be modest when you enter temples. Most temples refuse to allow people wearing shorts, short skirts, see-through dresses and other provocative types of clothing. If you are wearing transparent T-shirts or tank tops, have them covered with a scarf-like material when you are inside the temple.
You should be ready to accept the fact that certain areas of the temple are restricted places and entry is not permitted. Many temples do not allow anyone inside the Garbagraha or the sanctum sanctorum, the innermost chamber in which the presiding God resides. Only priests can enter that chamber.
In Kerala temples, all men should remove their upper garments above waist before entering the temple. Many Kerala temples even insist that men and women wearing pants are not allowed inside the temples and they should wear only local traditional dresses. These dresses are available for hire near most temples and you can avail that facility.