All in the title ,
Hello askubuntu , there is my problem :
when i try to access my apache website from my IP address i got the router homepage,
i'm using UBUNTU 12.04 LTS , Apache2 , and my router is Sagem F@st 1704
I assume you are trying to access the webpage using your external IP address, and not your LAN address (such as 192.168.x.x).
By default, the router will not pass anything to the LAN that it has not specifically been told to allow. It should have a place to allow certain ports to be passed through to the computers listening for that port, and often has easy configuration for common ports such as HTTP, SSH, FTP, etc. Web pages normally listen on port 80, by default, although this can be changed (but would require a special URL specifying the port).
The feature to allow connections to local computers is called "Port Forwarding". It is router specific, so you need to connect to your router and either find it, or look through the manual.
It's best not to allow all requests to be passed, but to only enable specific protocols such as HTTP on port 80.
If you're completely new to this, you might find this article helpful.
Edit - How to find the admin settings
In order to change the settings rather than simply viewing them, you must log in as "admin". According to the manual, the way to do this is to enter the ip address in your browser (192.168.1.1), and log in as "admin", with the default password "admin". But if this router is is from your ISP, they may have changed the password. If this is the case, you may need to search forums for your ISP to find the password, unless they will tell you what it is (good luck with that - they may disallow servers).