When you are telling, keep it simple. There are 3 parts to the message. Start by stating that you have epilepsy, follow this with some reassuring words and finish with practical information on what you want the person to do if you have a seizure. For example,
"I have epilepsy, it's not a big deal but it will be grand, just do ... if this happens."
The basic message can be adapted depending on the situation.
You can adapt the basic message to take into account your employer/school/college needs. You should try and set up an appointment to tell the necessary people or call them aside for a private discussion.
You can adapt the basic message in order to try and prevent the child feeling frightened about your seizures. Keep it simple based on the child's level of understanding.
You can adapt the basic message when you want to tell someone casually about your epilepsy, and in a way that strengthens your relationship with the person you are telling.