Light Switch Won't Turn Light Off
When a working switch ceases to be able to turn a light off, it is almost always a bad switch.


Turn the power off at the circuit breaker.
Remove the switch cover.
Take a picture of how the wires connect to the switch
Remove the old switch
Replace with the same type of switch, matching wires
Put cover back on
Turn power back on


This assumes that only one switch controls the light. As noted in the comment by @topshot, if there are other switches in the circuit, such as a 3-way or 4-way setup, one of the other switches could be the problem.

That said, the comment by @AndrewMorton is good advice. Most of us became comfortable working with electricity by observing and asking questions (in person) of someone who knew a lot more than us. If you can get a friend to assist who knows even a bit about electricity, it would be a good idea. And never take on any risky task when no one else is around to call for the ambulance.