I just done 1/2game with client and server on my local net but
now I want to understand if is possible to put the server side of game on an online shared server.
If you have done a client/server game it should be simple to do that. There really is nothing changing but that it is running over the internet and you can't use your local IPs to acces the server.
I want to know if I can use SFML libs on a game server. And Graph-Lib?
Yes, of course you can! For your server you normally don't need any graphics.
This is a strategic game with a very small use of TCP pakets but I think that here we have a bad ADSL line to do that in home.
Just check how much bandwidth your game needs in your local network and then check how fast your line is. For a game with many players you will need a computer with a fast acces to the internet though.
Where can I found info about small online server for my small game?
There are many tutorials about that I think. The SFML example/tutorials for networking will help you to understand how SFML handles it.
http://gamedev.net/ is always a good resource.
I hope that helps you