You have found some old coins that appear to be in pretty good condition and would like to find out how much they are worth. While rarity and condition are the two main factors affecting the price of any coin, an often overlooked third factor is popularity. For example, there are some rare half-cent, two cent, and three cent coins in excellent condition that will not bring as much money as a Morgan silver dollar of comparable rarity and quality because in the numismatics world the Morgan silver dollar is the most popular collectible American coin.
It might seem to be fairly easy to determine the rarity of a coin simply by looking it up in a book or even on-line, but rarity does not only refer to the date and type of the coin. Coins are also graded based upon their condition, so while a coin may have a fairly common date, that coin in near perfect condition may be very rare and hence worth a lot of money.