First hour
Move furniture, electronics and rugs out of the drip zone
Place buckets under active drips
Pierce a bulging ceiling with a small hole to release trapped water safely rather than letting it collapse
Isolate any nearby electrical circuits at the consumer unit if water is anywhere near a fitting
Photograph everything
Do not
Do not go onto the roof yourself. More people are hurt after a storm than during one.
Do not accept the first doorstep roofer who knocks. Use a local, established firm you can look up.
Do not agree to work without a written price and clear description of the repair.
Call a proper roofer
A local, insured roofer will make the roof safe quickly, document the damage properly for your insurer, and quote for a permanent repair in writing.