Normally, your landlord is legally responsible for repairs to the structure of the building: the roof, windows, doors, drains, gutters, baths, sinks, toilets, heating, hot water, damp and general building repairs.
Usually smaller jobs such as replacing fuses, or clearing a blocked sink are your responsibility.
An infestation of insects, mice or rats etc may be your responsibility or the landlord's responsibility depending on the cause and if your tenancy/licence agreement defines responsibility
You must also repair damage that you or your visitors have caused. Check your tenancy agreement to see whether it requires you to notify the landlord.