help please how to write chat program like yahoo messenger tht can allow people to register and chat between themself..

With sockets and more effort than you put into your post. (Albeit not much more)

i download some code.. it works only when i switch on my server..so i want to know how to make my server to be online without everytime to use my computer.. like how to put my server in internet???? please

That is a different issue.
Some server would need to host your program.
You can look into servers like
https://www.discountasp.net/
And other places suggested by GoDaddy (that host ASP.NET).