What can you do for a dog that is having a siezure?

If this is first time it's ever experienced seizures, best advice is to take it to the Vet so they can determine the cause (poisoning? trauma? tumor? other illness? epilepsy?). If the pet has a history of seizures and is on medication to control them, you may be able to get further advice over the phone from your Vet (or the local Emergency Vet)