I am a secretary and have heard rumours that my job is being advertised and have now seen applicants coming for interview. I need my job, what can I do?

How do you know it is your job that is being advertised? Maybe someone else needs a secretary.
Do you have any reason to think your management are unhappy with your work?

The best way to deal with this will depend on the company culture and the relationship you have with your manager.

If you can, approach your manager and simply tell him of your concerns. Choose a time when he is not under pressure, perhaps at the end of the day when everyone is winding down to go home.

You may find that there is nothing to worry about, this is an additional post, and nothing will change for you.
But you must be prepared for the worst. They may admit that they are planning to replace you. If so, ask why, and see if you could correct whatever the problem is with your performance.

Then you can either accept it, start applying for new jobs, or you can try to justify why they should retain you instead of replacing you.

So prepare these arguments before you ask the question. Evaluate your performance honestly and consider your strengths, and weaknesses.