Hi! I'm new to linux and I'm having trouble connecting to the internet.When I use the live cd I'm able to connect but after I installed it i could not connect. I use an ADSL connection that does need to login via PPPOE and has a dynamic IP. My connection goes like this:

ethernet[Cnet pro200]>Modem[Alcatel Home++500 ADSL modem]>internet.

I have tried the following and did not work:
-sudo PPPOE(just in case) it told me could not find access concentrator
-copy the configuration from etc folder

I'm not sure but I think I'm having trouble obtaining IP. I tried to read man ifup, ifconfig but I don't know how to use the user/shared/.../sample files.

I am presuming you modem has all the connection info such as user name / password etc and it makes the connection to the net. It assigns IP to the computers on you network and shares the net connection with all the computers.

check to see if you have an IP by

sudo ifconfig

if not try

sudo ifdown eth0
sudo ifup eth0

where eth0 is the network interface that connects to the modem

if this doesn't work please post the error message

"could not find access concentrator", I had this, too.
In my own case, I had to go to System >> Administration >> Networking, and disable, and re-enable the networking interface there to make pppoe work again. I had to do this after every reboot, often more than once, and it sucked big times.

Thanks for the time helping me. These are the messages I got from:
ifconfig
eth0 Link encap:Ethernet HWaddr 00:08:A1:9A:07:F3
BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:92 errors:219 dropped:0 overruns:0 frame:0
TX packets:12 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:6588 (6.4 KiB) TX bytes:2520 (2.4 KiB)
Interrupt:10 Base address:0xd000

ifup
^[[AListening on LPF/eth0/00:08:a1:9a:07:f3
Sending on LPF/eth0/00:08:a1:9a:07:f3
Sending on Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 6
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 9
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 18
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 10
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 10
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 8
No DHCPOFFERS received.
No working leases in persistent database - sleeping.

ifdown
Listening on LPF/eth0/00:08:a1:9a:07:f3
Sending on LPF/eth0/00:08:a1:9a:07:f3
Sending on Socket/fallback