I am workink in a company relating to computers maitenence and services I nedd to make its portfolio so basicaally I want the format how should I begin with can any one help out in this

A portfolio can and usually contains representations of the work that a person has done so far. Those could be pictures, writings of various kinds, letters of recommendation, etc.

Since you are describing a computer maintenance and services company of some kind, are you sure that you aren't looking to write an introduction letter of some kind describing the kind of services that you and/or the company can provide rather than a portfolio?

Thanks!