To be a secretary, you will need to have some education in the field. You could have done a diploma in secretarial practices, or its equivalent.
It might be necessary for you to know short hand in some cases. Computer proficiency, though, is a must anywhere.
As a secretary, your most important job is to handle telephone calls and screen calls as well. You need to be able to handle switch boards and also know how to screen and transfer calls. You will also need to be able to give ancillary support, so it is important that you have some knowledge of the function and know what it is about.
You will be responsible in overseeing all the activity of the department. This will include all correspondence coming into and going out of the department. You should also be able to co ordinate with other departments and liaise with them.
To be an executive assistant to a senior, you should be able to make up and follow the schedule of the senior. You should be able to fix appointments, follow their travel schedule, and also be able to monitor their whereabouts and be in touch with them constantly. You should be able to cover for them while they are away from the office, and also make out simple communications for them. You will also be the point of contact between the department and the senior while he is away.
There is a lot of room for growth. If you have the ability to pick up and learn quickly, you will be able to make it to much more than a mere assistant. You should be able to start learning more important aspects of the functioning of the department and also be able to multitask.
If you can handle all these competently, you might be promoted to become a part of the department, or even become an assistant to a more senior person, where you will no longer be at the level of an executive assistant, but enjoy a higher position and privileges.