This is a question which is not easily answered in one thread. There are many components to a local area network. At a high level... here are some of the things you need to consider.
Design the physical network. This includes cabling, patch panels, switches, routers
Design the logical IP network. You need to determine how many hosts will reside on this network so that you can design the IP scheme for your network hosts.
Design for Networking services. It is common to have networking services on a LAN such as DNS and DHCP. If you will have more than 20 Windows hosts, you should consider implementing an Active Directory Domain.
Sharing of Resources: Do you need to share files and other resources such as printers for you network hosts.
Is this network going to connect to the Internet? You would need to consider what type of Internet connection is required, bandwidth, service level agreements with your providers.
Will there be remote access required for users outside of your network. What are those requirements. Consider ISPEC and SSL-VPN solutions
Mobile devices are very popular now. How will you manage your corporate mobile devices. Consider an MDM (mobile device managemnet solution).
Security: Depending on the security requirements, you may need one or more firewalls to protect different subnets (zones) within your network. How will you protect your hosts from malware? Central Antivirus soution? What about your patch management strategy?
Will you require to host a website for external users? Hosting the website with a provider vs hosting within your LAN?
This is just a subset of some of the things you need to consider. Do you have any specific question about a LAN?