i enterd static ip in modem and connect my 3 computers to that modem with automatically confic of system ip so now i want to access my one computer through any where via internet please let know how i do this
You mentioned a modem, but do you have a router connected to the modem? If the ISP provided this "modem", now they typically provide you with one box that serves both functions.
In any case, what you really want is that on the router, look for the "port forwarding" or "NAT" configuration.
Say you want to be able to access an workstation on the internal network... on your router, you would create a port forwarding rule that states... on port 3389 on the public WAN port, forward the traffic to the internal IP of the target workstation.
that's basically it. You just match an port number on the WAN interface (public IP) to that of a port number listening on a computer within your network. Oh and you have to make sure that if you are running a local firewall on the computer on your private network, that it allows traffic on that port.
Then from the internet, launch the application that you will use to connect back to the computer in your network and either provide the fully qualified domain name of the WAN IP of the router, or just type in the IP address of that WAN interface.
Logmein is a nice answer and does not require opening a port on your router, but is scheduled to go away.
I would open the router port, forwarding it to your machine. If you can handle it, I would use a different port than the RDP standard (3389), as RDP is not terribly secure. Running it on the standard port may open you to attack.
If you change the port, I would change the RDP listener port on the PC, then forward that port on the router.