Dear Preeti,
First of all find out whether employees database is available with company or not? if yes then upgrade the same and if not then start with this first. Collect all credentials of your employees, maintain database or their personal files which will help you in future.
Secondly prepare Organizational chart both for HO n site. Ask HOD/ concerned site incharge to send you the necessary detail and accordingly you prepare it or you can even ask them to send you the chart. This is what we do in our Company. Even i am from construction company and we have approx 45 site all over India.
Then prepare format of Manpower requisition, Offer Letter, appointment letter, transfer letter, termination letter etc. whichever you feel is required from company's point of you but consult your MD/ Sr. for this.
Job Description of Technical/ Non technical staff. For that you need to consult concerned HOD/ Site Incharge. Which will help you in recruitment process for finding right candidate.
As mentioned above there are other aspects also you can start with like Induction, Confirmation, Performance Appraisal, welfare activities for employees etc.
Preeti i feel you are too fresher for all these activities, and to form a Dept. your company need to appoint Sr. level person. You can do this but being fresher you need guidance from seniors so you prepare action plan for your dept. and have word with your MD. If possible suggest him to appoint Sr. level person, under which even you will be able to learn many things which will definitely help you for your career.
All da Best ..