First of all, relax and sit down as comfortably as possible. It is better to take off your shoes and massage your feet a little to increase blood flow there. If you are worried about a headache, then a light massage of the temples will be beneficial. Be sure to ask the guide for a glass of water. And of course, in case of deterioration in health, immediately inform the crew. The plane may need to take a lower altitude to get better.
For a start - inform the flight attendant about it. Further according to the situation: from taking any medications for non-serious cases to an emergency landing of the plane with subsequent hospitalization.