Libraries Try your local public library, especially one with a reference section. They will have access to many good coin books, and other aids to identification. Don't expect the librarian to be able to identify and value your coin, but they can help you find the right books. It may sound obvious, but the first stage is to identify your coin, only after this will you be able to find its value. The most important aspect affecting the value of a coin is often its condition, otherwise called its state of preservation, or its grade.
Most collectors prefer their coins to be in mint condition, or at least as close as possible.
Even quite rare coins may be almost worthless in worn condition. Once you know what the coin is, you can start to work out its value. Apart from its age, date, type and condition, supply and demand are what will dictate the price at which a coin is likely to change hands. Do you mean a buying price or a selling price. Don't forget a dealers selling price would be your buying price, and his buying price would be your selling price.