How to send message in LAN environtmnet

Hi all,
I really need to know how I can send message to all workstations if I really need to boot my server or if I need to work on our servers. How I can send instance messgae to all the users in a LAN environmen. We do have tcp/ip netwroking here.I am junior here , dont know much about network administrative tips. Is there any webside where I can learn stuffs to be professional like you guys. How about if I wanna access to a workstation from my desk. I do have administrative passwrod and everything.Just need to know a lot. Please advice.thanx a lot.