To boil liquid, the saucepan needs to go on medium-high heat until lots and lots of bubbles rush to the surface. At this stage, the liquid is very hot and in the case of water, is 100°C. Food is rarely cooked in boiling liquid for long; normally it is brought to a boil and then reduced to a simmer.
To simmer liquid, bring the water to the boil first and then turn the heat down until there are just tiny bubbles coming up to the surface. This is much more gentle than boiling and lets the food cook more slowly, bringing out more flavours, and it helps to stop it from breaking up.
CAUTION! Always ask an adult for help when using the stove.