The proper transmission fliud is usually listed in your owners manual. For the most part a DEXRON/MERCON will do for any vehicle unless its one of those exotic types in which case you probably have the money to take it to a shop and pay them 85.00 per hour to top up your transmission, Older cars and trucks sometimes use the different fluids such a TYPE 'F" fluid for Fords....best deal is see a Auto Parts store.
It is critical that you use the right one. For example, newer Chrysler/Dodge products call require ATF + 4 fluid, (I think it may have gone to ATF + 5) If you transmission called for ATF + 2, ATF + 3 then ATF + 4 is acceptable. If you use different fluid in it the doesn't have the correct chemical composition, you can cause problems. You can buy this fluid at the dealer and add it yourself. Aside from the vehicle specific question, there is the question if you have a automatic or manual transmission. They differ. Also pay attention how your particular oil level is checked, some are in park and some are in nuetral-it will matter.
While Chrysler/Dodge often calls for ATF4, older C/D vehicles should use the type of fluid that is stamped on the trans dipstick. Using ATF4 fluid in a unit that calls for Dexron WILL!! cause severe internal damage! I know, it happened to my 86 Dodge van.
Here is a list of different fluid types - there are more, but these are the most common. You must ASK a reputable source to be sure to get the correct type:
Type F - Introduced by Ford in 1967 for their automatics. Also used by Toyota.
Type CJ - Special Ford fluid for C6 transmissions. Similar to Dexron II. Must not be used in automatics that require Type F.
Type H - Another limited Ford spec that differs from both Dexron and Type F. Can be replaced with Mercon.
Mercon - Ford fluid introduced in 1987, very similar to Dexron II. OK for all earlier Fords, except those that require Type F.
Mercon V - Ford's newest type, introduced in 1997 for Ranger, Explorer V6 and Aerostar, and 1998 & up Windstar, Taurus/Sable and Continental. Must not be used in 1997 or earlier Fords.
Dexron - General Motors original ATF for automatics.
Dexron II - Improved GM formula with better viscosity control and additional oxidation inhibitors. Can be used in place of Dexron.
Dexron IIE - GM fluid for electronic transmissions.
Dexron III - Replaces Dexron IIE and adds improved oxidation and corrosion control in GM electronic automatics.
Dexron III/Saturn - A special fluid spec for Saturns.
Chrysler 7176 - For Chrysler FWD transaxles.
Chrysler 7176D (ATF +2) - Adds improved cold temperature flow and oxidation resistance. Introduced in 1997.
Chrysler 7176E (ATF +3) - Adds improved shear stability and uses a higher quality base oil.
Genuine Honda ATF - Special ATF for Honda automatics.
Toyota Type T - Special formula for Toyota All Trac vehicles and some Lexus models.