There is a really wide selection of restaurants on the island that serve almost any cuisine that you can think of. Several English Restaurants can be found in Georgetown and in Batu Ferrenghi. Local Malay food is spicy, and very tasty, but you can also get Thai, Indian and Chinese food. Prices for eating out vary depending on the quality of the restaurant, but on the whole are very reasonable.
Typically you do not need to make a reservation except on weekends, but it is usually a good idea to get the concierge at your hotel to call in advance in order to check on your behalf. More expensive restaurants will almost always add 10% onto the bill as a service charge, but in a cheaper place, a 5-10% tip is normal if the service is good.