Fitting a timer extractor fan can be done in two ways.
The most common arrangement is so you turn the light on, fan comes on. Turn light on, fan continues running for a pre-determined time.
You COULD have the fan seperate from the light - so you turn on a seperate switch to turn on fan, then turn that switch off and again fan continues running for x minutes.
Having the timer fan to activate when the light is turned on is the most usual way (it's the only way people won't forget, assuming it's dark outside).
To do this you need to find a point in the bathroom light wiring that consists of permanent LIVE, switched LIVE, NEUTRAL and EARTH.
This may be at the light, or at the light switch, or at a junction box - house wiring can vary a lot.
You will also need a 3 pole fan switch - typically a wall switch outside the bathroom, or a pull switch in the bathroom. This 3 pole fan switch is for maintenance. For normal use this switch remains on all the time. However, it's useful for turning the fan off at night to prevent disturbing people at night when going to the loo.
Wiring from the supply, via a fan switch, is typically done in 3 core and earth flat cable.
How does your current fan get it's supply? Is it activated by a seperate pull cord or something?