A renaissance fair is a gathering of people who are re-acting the Renaissance, a period of religion rediscovery. It mostly consists of knight. Alot of people go their to eat traditional food, and get small trinkets.
Get a long skirt, a blouse with puffy sleeves, and a lace up vest. These need to be in basic colors if you want to look authentic. EBay has a section where they sell/bid on items made for Renaissance...
There really is no 'official' RS renaissance fair, probably 3rd party groups. But there is 'RuneFest' which is a gathering of the most dedicated(and not so dedicated) players who'd like to attend...