1989 Honda Prelude starting problem?

I have a 1989 Honda prelude automatic 2.0 si i have been having starting problems my car will start then when i go into a store and come back out it wont turn on i have to sit in the car for up to a half hour til it will turn on at times it wont even turn on when it hasn't been turned on in a while i heard it may something called the main relay switch what is that and were is it at in my car? How much does it cost and is it something i have to take in for someone to put in or can i do it and how long does it take can you also tell me what it looks like so i know what i am looking for? When calling for the part what is the correct term is in Main relay switch? do you happen to know how much it cost? i want to thank you for you help