Under the Bonnet
Here is a view under the bonnet of my Ibiza. The four main items to identify are arrowed:
Regular checks will help you to spot a problem early, before it causes a breakdown.
Engine coolant
NEVER take the top off the coolant bottle when the engine is hot - the coolant is very hot and under pressure, there is a risk of severe scalding. Wait until the engine is cold, and cover the cap with a cloth when removing it.
You can see the pink coolant liquid in the coolant bottle,and the minimum and maximum marks. The liquid is a mixture of water and anti-freeze. Top up with water and anti-freeze, to prevent the mixture becoming too weak. Don't splash the anti-freeze, it acts as a paint stripper.
Any significant drop in the coolant level can cause very expensive problems to your engine. It could also be a symptom of the head gasket failing, rather than a simple leak. Keep it checked.
Windscreen washer
Check the windscreen washer by taking the top off and looking in - very high tech! Top up with water and washer fluid - available from any car accessory store. The washer fluid will help prevent the liquid freezing, as well as keeping your screen sparkling.. NEVER put anti-freeze in the washer bottle - it acts as a paint stripper!
Oil
Oil is one of the most important parts of your engine - without it everything will seize up! (Very expensive). Park the car on level ground, wait a few minutes for the oil to drain down into the sump (the bottom of the engine), then remove the dip-stick (the orange thing at the bottom of the picture).
Wipe it clean, and look for the markings at the end to show the maximum and minimum marks. Put it back, remove it again, and you will see the oil marking the level, hopefully near the maximum mark. If you have to top up the oil, look in your car's handbook to make sure which type to use (yes, there are lots of different types of oil), and only put in a little at a time. Over filling oil can result in oil seals failing - more huge expense. It can also damage your catalytic converter - if your car has one.
Brake fluid
Brake fluid makes the brakes brake - without it you won't stop. As you can see the reservoir is hidden away on my Ibiza, but it is a transparent container, with minimum and maximum marks. Any significant change in the brake fluid reservoir needs urgent attention. Brake fluid also acts as a paint stripper.