i want to know how to write a design for the small sized network for not more than 80 users

Click Here there are plenty of useful links from that search

You would need to provide more details for guidance. How many locations, how many computers, types of computers, access required (wired, wireless). How many subnets are required? Is there a need to seperate network traffic into different vlans? Any other security requirements (perimeter firewall, secure zones, remote access, etc..)?