Coin collecting is a hobby that can also become an investment over time. Some old coins grow to be worth much more than their initial face value. Others never become worth much more than the value of the base metal or the stamp on their face. Knowing how to find out how much old coins are worth is an essential skill for the serious coin collector.
Make an inventory of all the coins in your collection. List the country of origin, the value, the year, any special marks and the condition of the coin.
Visit the Professional Coin Grading Service website at PCGS.com. This company provides the gold standard for pricing in the coin industry. Look up the value of your coins individually to see how much they are worth according to PCGS. Look up coins in the "Red Book", which is the "Guide Book of United States Coin" for another reliable price guide.
Investigate what the coins actually go for in the real world. Go into local coin shops and see if they have versions of the coins you want to place a value on for sale and check what they sell them for. Also, check sites like auction sites or speciality coin collector retail sites. Sometimes the appraised value and the market value don't line up on any collectable, and coins are no different.
Take your complete collection to an antique or coin collector's shop and have the entire collection appraised. Sometimes a complete collection with many collectable coins can actually be sold for more as a collection than as individual coins.
Organise your collection well before you try to sell it or have it appraised. A box full of old coins isn't going to get much attention from collectors, but a neatly organised binder of coin sleeves with coins all in the appropriate places will.