The first thing is to prevent the patient from injuring themselves or others. The safest place to be is on the floor, so try to help the person to lie down gently on the floor or at least get onto a safe piece of furniture. You should also move all nearby hard objects, chairs etc out of the way, as the patient may make uncontrolled movements and hurt themselves by knocking into something.

The patient should by lying with their mouth pointing down, to prevent injury to the tongue, though if they vomit it would be safer to turn them on their side. As soon as the person is safely lying down, you need to call for medical help. Keep watching the patient and don't leave them alone at all. Note everything they do as the medics will need to know as much as possible.

You can find more information on the causes and treatment of seizures here.