The engine and transmission should be at operating temperature, on level ground and the transmission in Park. Pull the trans fluid dipstick, wipe it off, re-install and recheck the dipstick. Add fluid through the dipstick tube to bring the level up to the full mark. Add in small amounts, do not overfill!
You may need a funnel, and make sure you use the correct fluid (very important!). Check you owners manual for the correct fluid type.
if you dont have a trans dipstick you must have the newer style trans pan. the fluid is filled from the large plug on the pan with the car "WARM NOT HOT!!!" a tester is most accurate, spec is 35c. dont use generic ATF its special stuff! go to the dealer. pretty sure your service interval is every 45,000 miles.