U.S. Federal Reserve Notes are the primary paper currency of the United States, and are generally worth their face value as money within the country. They can also be exchanged for other currencies virtually anywhere in the world. A few Federal Reserve notes have extra value as collectibles, such as some made in the earlier decades of the 20th century, or more recent notes with printing errors.
Familiarize yourself with the basics of Federal Reserve Notes. All of them, without exception, will say they are Federal Reserve Notes on their front, regardless of when they were issued, their denomination or any other design elements. At a glance, they're distinctive because they feature green U.S. Treasury seals and serial numbers--except for the larger bills issued before 1928--as opposed the red seals and numbers of United States Notes and the blue seals and numbers of Silver Certificates.
Organize your notes by denomination, series year and letter, and condition. Denominations mostly include $1, $2, $5, $10, $20, $50 and $100, with larger ones printed before 1945, including $500, $1000, $5000 and $10,000--none of these circulate any more and can be quite valuable. The series date and letter are found on the front of the note, though sometimes there is no letter. The most valuable notes are generally "uncirculated"--crisp and not worn at all. Others are called "circulated," which indicates various states of wear. They are usually less valuable to collectors.
Check for certain details that might make your notes more valuable as collectibles, such as a star in the serial number, a printing error or an "interesting" serial number. Star notes were issued to replace regular notes damaged during printing. A printing error can be any number of mistakes on a note, such as cuts in the wrong place or print that's been left off. "Interesting" serial numbers are a minor specialty among paper money collectors, and include low or high numbers, such as A00000019A or B99999999B, or other oddities, such as D77777777D.
Consult a standard reference to estimate the value of the particular notes you have, such as "A Guide Book Of United States Paper Money: Complete Source for History, Grading, and Prices" (otherwise known as the Red Book). Or look on currency dealer's websites or auction sites for a general idea how how much a note will be worth if sold as a collectible.
If you have something that might be particularly value, ask a coin/paper money dealer--or two or three--look at it and give you an estimate. They might also make you an offer for it. Most Federal Reserve Notes, especially circulated ones, are worth very little over face value, however. Yet the are exceptions. Series 1934 to 1934D notes in flawless condition, for example, might command two or three times face value. Higher, obsolete denominations might fetch even higher multiples of their face value.