Have you Port Forwarded? (Opened The Port In Your Router?)
You must Make sure the Port is open, and not blocked by your..
Router Or Firewall.. Here Are some tutorials..
1) http://www.infohub.me/showthread.php?t=205
2) http://techunwrapped.com/2011/01/09/tutorial-create-a-minecraft-multiplayer-server/
3) http://segmentnext.com/2010/10/17/minecraft-private-dedicated-server-guide/
Port Forwarding) http://portforward.com/ (Choose your Router.
The Ram Error Is Common And Nothing to worry about.. But..
The More you Explore = The Bigger The world = The More Ram It Will Consume.
Hope it helps :)
Cmd = Command.
Start menu, Run.. Type.. Cmd.. Press Enter.
Edit:
Thats' the Problem.. It Requires alot of Learning to Bypass a Router..
A Successful Server is not so Easy to set up Behind a Router Due to..
The Router's Firewall/Security. Thats why we must Port Forward.
Goto start menu, Hit run, type.. cmd.. and hit enter.. then yupe.. ipconfig..
And hit enter, look for the Ipv4 Address 192.***.0.* .. that is the Computers IP.
So when you Login to your Routers Configuation you must Open a Port For your IP.
Dto do this you must look on the router for its model and look up how to access..
Your Routers configuation.. Something Like.. USERNAME: admin PASS: sky
Try This Guide -> http://www.minecraftforum.net/viewtopic.php?f=22&t=12559