How do we send message in a lan network in NT operating system using dos prompt?

Use netsend <name of computer>"message" syntex

Binod_dhungana is right how ever the way he told you would most likely get you more stuck the proper usage of net send is:

NET SEND *name or IP of computer (if you want to send it to every one put *) * *message*

Exampel:
Net send server Hello

Explanation:
Typing that out in command prompt would send the message "Hello" to the computer called server and would cause a text box to appaer that said "Hello" along with the time, date, name of user that send it and the name of the computer the message was sent from.