Place your car at the level spot. Stop the engine. Wait for a while to let the engine oil to pour down to the oil pan. Pull the engine oil dipstick.

 If you don't know where is the engine oil dipstick, check your owner's manual, usually it has a bright handle saying "engine oil".

Wipe it off with a clean rag or tissue. Then insert it back all the way down into its place.

NOW, pull the dipstick again and check the oil level. Normally it should be at "FULL" mark. For example, here you can see that it's a bit lower. It's not a big problem yet, but it's better to top it up. Check the oil condition: If it's way too black, it's definitely time to change it. If it's slightly-brown, it's O.K. If it's dark-brown, but still transparent, it's admissible but it's better to change it soon. 

If it's white (coffee with milk color) it means the engine coolant mixes with the engine oil because of some internal engine problem, for example, blown head gasket - have your car inspected.

How to top up the engine oil: 
It would be better to add the same type and brand of the engine oil as you already have in the engine. Add a little amount of the oil as it's shown in the image. Wait for a minute to let the oil to pour down. Check the oil level again with the dipstick. If it's still low, add some more. But don't overfill it. Don't forget to install the dipstick back and close the oil filler cap when you finished.

Place your car at a level surface and engage the parking brake. Start the engine. Set transmission shifter in "P" (Park) position, and let the engine idle (on some cars this procedure may be different, check the owners' manual for details). Pull the transmission dipstick. Check your owners manual to find where transmission dipstick is located in your car.

 Wipe it off with a clean lint free rag. Then insert it back carefully all the way down into its place.

Pull again and check the fluid level. If the engine is cold, it should be within "COLD" marks. If the car was driven and is fully warmed up, the level should be at the upper end of the "HOT" mark. If it's just a little bit lower I wouldn't worry about it. Otherwise I'd top it up. Check the fluid condition also: If it's too black and dirty with burnt smell - your transmission is not going to last. Normally it should be clean and transparent, as in the image. The new fluid comes red. Over the time it becomes brownish. If it is brown, check your owner's manual, may be it's time to change it. Some manufacturers require to change the transmission fluid at 30,000 or 50,000 miles others specify that you never have to change it - check what's your car owner's manual says.  

 How to top up the transmission fluid: