Real estate agents can help with selling your house by advertising your house and by helping walk a home owner through the sales process.
Yes, by contacting a local real estate agent they should be able to give you an idea of what kind of price you can expect to list your house for based what similar properties having been selling for in your area.
There are two main methods of selling a house available: Private treaty and Auction. Private treaty is where the property is listed for sale and you receive an offer to purchase the house and you either accept or decline the offer. An auction however involves marketing the property and having a set auction day where interested parties bid against each other to win the property.
Yes, most real estate agencies will also have a rental side of the business that deals with managing rental properties from finding tenants to managing issues that might crop up with the property.
Friends and family will have often dealt with real estate agents in your area and be able to offer suggestions as to who they have had either good or bad experiences with. Online reviews can also provide some usual information providing you can trust the reviewers.