Humidity sensing fan installation
You'll have to check the light fixture and the switch to see where the hot wire is.  It can be in the ceiling, with a wire running to the switch to break the circuit, or at the switch and a wire running to the light to power it.  If the hot is in the ceiling, you are in luck.  You can tie into that for power.  If the hot wire is at the switch, you'll have to run a wire down the wall and into that box.  Not impossible, but not easy either, it will be much easier if its an interior wall.  I am an amateur and have done it many times.

I would suggest cutting the power at the breaker, pull off the light fixture from the ceiling, and the switch from the wall.  Then turn the power back on and with the light turned off, carefully probe the wires with a contactless voltage meter to see where the power is coming from.