Others have given some good answers already, so I'll just simplify a bit. Here are the "rules of thumb." All modern currency is worth face value, so it's important to know where that "modern" line is. All dimes, quarters, half dollars, and dollars minted in 1964 and prior will (currently) be worth about 20x their face value due to being 90% silver. A 1962 quarter is worth about $5. A 1962 dollar is worth about $20. These values change as the trading value of silver goes up and down (and also suggests that our currency was worth about 20x as much in the 60's as it is now). Nickels have always been relatively unimportant, unless you get a special one. The same goes for pennies, but pennies (minted in 1984 and prior) are now worth about 3x face value for copper content (100 pennies is $3 in copper, though it is still illegal to melt them down). All modern pennies are made of zinc with only a tiny bit of copper.
These guidelines that I've mentioned work as a good foundation, but then there are special series, mint errors, short runs (only a few coins of a certain type in a certain year came from a certain mint), and stuff like that. To get more than these general guidelines, you need to start learning individual coins and series. It's good that you have a place like Tnet where there are a bunch of people interested in the same thing.