If you are talking about an automatic transmission then you will need to raise and properly support the vehicle if you can not get under it.
If you locate the transmission pan you will see two bolts on it, one directly on the bottom in the center of the pan. This is the drain plug. The next bolt is located slightly higher oon the pan one one side. This is the overfill drain plug. This plug is used to fill the transmission.
Remove the transmission pan overfill plug and if no fluid comes out then you will need to buy some dextron (r) 3 transmission fluid and you will need to buy a fluid pump. You can use the same fluid pump that is used to fill the manual transmiosn with.
Fill the the fluid thru the over fill plug untill it spills out, as soon as this happens replace the plug. Run the engine on idle untill it gets warm, then stop the car and check the overfill plug once more for accuracy. If the tranny needs more fluid then add some more untill it starts spilling then again replace the bolt.
The only time that you need to replace the tranny fluid is when you have been driving off road or when you have been doing repeated short trips, driving rough roads or pulling a trailer.
The transmission holds a max of 9.1 quarts when empty completely, includes an empty torque converter.
If you have a manual transmission then you need to buy some 80-90 gear oil, locate the tranny and somewhere on the side there will be a big fat square bolt. This bolts needs to be remove and filled withe the gear oil untill it spills out. When it does then simply replace the bolt and you are done.