Coin collecting (numismatics) is one of the most popular hobbies worldwide. This guide is intended to assist beginners to the hobby. Like all collecting, knowledge is essential to your success and satisfaction with your collection. Unlike most collectibles, coins have been extensively cataloged and numerous references are available to determine the value of almost any particular coin. A beginner in antique furniture doesn't know if that dirty old wood chair is worth $100,000 or almost nothing, and there is no easy way to figure it out. With coins, one can just look it up. You need to know the country of origin, the denomination, the date, the mintmark (usually a single letter, indicating the city where the coin was made), and the grade (condition).
Before spending more than a couple of bucks on a coin, the beginner should have a basic understanding of coin grading, and should consult one of the many references. This is the best way to avoid paying far in excess of the coin's actual value.
Grading can be difficult for beginners. The American Numismatic Association (ANA) has published grading standards for U.S. coins and these standards are widely applied to foreign coins as well, although collectors in some other countries use different standards and terminology. In addition to the ANA grading standards, A Guide Book to United States Coins (commonly called the Red Book) provides brief grading descriptions, and Photograde provides pictures of various U.S. coins in various grades. A trip to a coin show, or a bricks and mortar coin shop, allows the opportunity to see thousands of coins in various grades and is invaluable in gaining grading knowledge. One final note: grades apply to undamaged coins. Holed, gouged, scratched, bent, corroded, and cleaned coins are worth a small fraction of undamaged coins in the same grade. Always check for these defects, and never clean coins.
For non-U.S. coins, the Standard Catalog of World Coins is the most widely-used reference. As big as a phone book, each covers a century (17th - 21st) and includes every government issued coin from every country on the planet. Digital versions are now also available. Neither is inexpensive, and new versions are published every year. However many public libraries have them, and old editions can sometimes be bought for much less than new. The values usually do not change radically from year to year. Foreign coins are less in demand than U.S. coins, and can frequently be bought at a larger discount to catalog value than U.S. coins.
There are numerous valuation guides available for U.S. coins. The Red Book, and monthly "Coin Values" and "Coin Prices", and others, all provide retail estimates of value. The Red Book is published yearly, and in volatile markets can be out-of-date. "Coin Values" (sometimes referred to as "Trends" because of its former name "Coin Value Trends") generally has the highest prices, "Coin Prices" is generally lower. Coin dealers will not pay these prices for your coins. These publications provide a range of values that dealers and shops typically ask for coins. The "Coin Dealer Newsletter" (often referred to as the Greysheet) is published weekly, and provides wholesale (dealer to dealer) prices. Dealers and shops might pay these prices for your coins, if they are not common low-value coins they are overstocked with.
The point is this: Use some sort of valuation guide, and do not buy over-graded or damaged coins. This is all you need to know to avoid paying $50 for a $15 coin (or $500 for a $150 coin). There are counterfeit coins out there. If your knowledge is limited, have a trusted experienced collector or dealer check it out.