If you do not know how to treat the individual who is undergoing a seizure, the best you can do is cry out for help or call a doctor.
The main idea is that you remain calm and composed and make sure that you do not freak out people around you. Take charge and call out for help.
There are different kinds of seizures, so you should make yourself aware of the type of seizure the individual is having. This way you will know exactly what to do and also keep the situation under control.
Detect the situation quickly and prevent the individual from falling on the ground to prevent him from getting any fatal injuries. A blow to the head could cause a concussion, and that can definitely add to the complications.
Check if the person is breathing.
You need to understand that the individual will be doing involuntary movements that he will not have control of. Do not panic and check if the individual is breathing, if you do not know how to perform a CPR then call a doctor quickly and get his help.
If you think that for the doctor to reach the place will take time then call up a doctor and tell him to guide you.
If the person has come out of the seizure and has regained consciousness then give him some breathing space. Tell the people surrounding you to step back and make some room for the person to breathe.
Put him in a sitting position and keep on talking to him till he gets on his feet. After he has done so, take him to the hospital to seek further help.