Whether you want to try to fix your transmission yourself or take it into a repair shop, being knowledgeable about what is going on in your vehicle can save you money. If you have some idea of what your transmission problem is, you will not be duped into paying for more services than you need, and it may even be a simple fix you can take care of yourself.
The first thing you should always check is your transmission fluid level. Low fluid can make the transmission shift harder and eventually cause serious transmission damage. Also check that your fluid is a nice red color, and not brown or grey, which can indicate that the fluid is contaminated. If you suspect your transmission may be leaking fluid, it is fairly easy to determine where it is coming out of, since transmission fluid is a bright red color. Check everywhere that your transmission connects to the engine and selector shaft, the drain hole underneath the transmission, and around the speed sensor. If your transmission has a cooler, the leak could also be leaking transmission fluid into your radiator. Transmission fluid is an oil, so it will not mix with the radiator fluid; just check your radiator fluid to see if there is any red stuff floating around!
Even in vehicles with automatic transmissions, many cars give you the option to shift manually, even if only to low or second gear. If you have been having transmission problems such as hard shifting or delayed shifting, try to manually shift gears a few times while driving. If there seems to be no problem with the manual shifting, the problem is likely with the transmission controls and not with the gears themselves. This can help you narrow down the field of potential issues by eliminating anything to do with the gears.
For this step, you will need an automatic transmission gauge, and the service manual for your transmission or vehicle. The service manual can tell you where to check the pressure (there should be several places) as well as what the pressure should be at each location. If the pressure is wrong, the transmission pressure solenoid may need to be replaced, a repair that requires a partial disassembly of the transmission. A bad pressure solenoid can lead to transmission problems such as hard or erratic shifting.
This is a little more complicated due to the equipment you need, and may be cheaper to have done by a shop. You will need a computer diagnostic tool, also known as a code scanner. Automatic transmissions are now, like many other vehicular systems, largely computerized. This means the computer may be able to tell you what your transmission problem is. Once you have the scanner, you just hook it up to the transmission and it will give you one or more codes. You can look those codes up to find out what the computer thinks the problem is.