We live in the 21st century–each and every secretary works with computer on a daily basis. A great applicant for this job should have some knowledge of MS Office (or similar software they typically use in your country), including MS Outlook.
You should backup your knowledge with some experience, explaining your work with MS Office in the past. And you should also emphasize your efforts to improve your computer skills.
I can work with both Word and Excel, and also with MS PowerPoint. Actually, I have been working with these programs on a daily basis for the past five years. In MS Excel, I can create charts and work with simple functions. In MS Word, I can do pretty much everything.
When preparing reports in my last job, I worked with MT Office on a daily basis. On the top of that, I am very skilled with MS Outlook and with the internet. I would give myself 9 out of 10 from both Word and Excel, if I were to score my abilities.
Practical test in an interview
Some HR managers will test both your skills and trustworthiness with a simple exercise. There will give you a sheet of paper with some text that includes headings, bullets, and formatting, and they will ask you retype the text to MS Word, with the correct formatting. They may even ask you to do some simple calculations, or prepare a graph in MS Excel.
Doing so, they can see how quickly you type, if you can find what you need in Word or Excel, and so on.
Having this option on your mind, try to not overrate your skills. Tell them less, and surprise them in a practical test. That would present you as a humble and trustworthy job applicant–and most HR managers love such job applicants. Not only for a position of a secretary, but in general.