Turn it off and disconnect power
Shut down normally if it responds; otherwise hold the power button until it turns off. Disconnect the charger and every accessory. Do not restart it to see whether it still works.
Let it dry without aggressive methods
Do not reconnect cables until the Mac and accessories are completely dry. Leave it in a dry place with airflow for at least a day.
No heat, compressed air or rice
Do not use external heat, compressed air, cotton swabs or paper in ports. Do not put the MacBook in rice.
Record what happened
Note the liquid, time, whether it was powered and current symptoms. Do not open it at home; these details help the inspection.