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.