Performance Management system is one of the key tasks of HR Management, now that you are beginning to introduce one, much careful thought needs to be gone into. It would depend on the comprehensive knowledge of the organisation, its nature, operations, background, levels of personnel, linkage of the PMS with administrative aspects like pay, increment, promotion/ demotion, transfer etc.
Is this going to be your first year of operation or is there any such practice in the organisation?. If yes what is the feedback about it? There is no standard formula of PMS which fits all the categories. You need the services of a consultant for evolving a well thought out and effective PMS.