Basic car maintenance for Dummies

The thermometer light is showing so I need to top up with oil and water asap.

Question about water to use - does it have to be 'distilled' (i.e. boiled & cooled) or can water straight from the tap be used ?

It should be coolant - not water. Coolant protects engine from corrosion and stops the pipes bursting in very cold weather.
Best ---> Worst
Coolant, Distilled water, boiled filtered water, tap water, drain water.
Use right viscosity of oil to top up too; listed in manual.

Agreed, but if the temperature gauage is going into the red, you may have a blown head gasket.

Let the engine cool totally.

Top up liquids as advised.

If it's using coolant suspect a leak or blown h/g.

To check for a blown head gasket open the oil filler cap and see if what is inside is black looks like oil ( should be ok) or if the is an off ****e gunk (head gasket is in trouble).

Easy enough to spot the difference.