Engine oil
Too little oil and your engine will seize up whilst an excess could lead to a blown gasket. Check the oil level regularly and top up, if necessary, until it reaches the desired level. If the oil on the dipstick is black, it needs to be replaced. If it's light brown and emulsified something is seriously wrong and you should take your car to a garage.

Water levels
You shouldn't need to fill up the radiator that often. However, if you need to check your water levels remove the cap carefully! Under NO circumstances should you do this when the engine is hot.
Before topping up the levels, check the required mixture of antifreeze and water needed.

Fluids
Brake fluid leaks are dangerous. It is crucial that you find the fluid reservoir in your car and check the level regularly. A sign that your brake fluid levels are off is a 'spongy' or 'soft' action on the brake pedal. If you have power steering or an automatic gearbox, you'll also need to check their fluid levels. On a manual car there'll be one for the clutch. If you need to top them up, ensure you're using the correct fluid. Find this in the car's manual.

Windscreens
Although chips or cracks in the windscreen might seem insignificant, they can get worse over time and cause you to fail an MOT. A rule of thumb is that if you have damage that is over 40mm in diameter than you should get your windscreen changed.

Lights
It might be obvious but many people are driving vehicles with defective headlights, brake lights and indicators without being aware of it. Park your vehicle pointing at a wall and then activate all the lights in succession. From the reflections you should be able to see if any are broken. Repeat this process with the rear of the car facing the wall to check your brake lights as well.

Tyres
Under and over inflation of tyres not only affect your grip on the road but the efficiency of your brakes and wheel alignment. It also causes tyres to wear out much faster as well as increasing the risk of blow-outs and handling problems. Tyres should be checked at least once a month and inflated to the pressures indicated in your manual.