Unplug your appliances, especially kitchen stuff, kettle, toaster, washing machine, etc. Switch off the immersion heater if you have one, and the cooker switch. Put light switches to OFF.
On the panel where the RCD is switch off all the associated individual circuit breakers.
Will the RCD now reset? Does its test button work? If either NO you need expert help.
If both YES, then with the RCD ON, switch on individual circuit breakers, one at a time.
If switching one of them causes the RCD to trip again you have narrowed the search to that circuit. You probably still need help, but you can use everything else in the meantime.
If all the breakers are now ON and the RCD is still holding ON, then one at a time plug in and operate each of the appliances you unplugged. Obviously if one trips the RCD it's faulty; repair or replace.
Check the fixed items, cooker, etc work, try all lights. ( HINT Outside lights are particularly suspect.)
If all the breakers stay on then just carry on as normal but note what you are doing if it trips again.
If the problem keeps randomly recurring, and you can't identify it to any single item, then you need an electrician.