you should be more specific , are you trying to go wireless or wired?
what type of network adapter...router,etc
my guess is that you are not sure how to setup your connection in ubuntu.
if your running dhcp from your router, and you not wireless it should just work, providing ubuntu has your nic driver, find out the brand and google
ex " intel pro 100tx ubuntu" and see if it is supported, wireless is a whole other story, i use ubuntu, and it works great but there is a slight learning curve. don't give up to easy
are you on wireless? if so did you select the wireless network you want? i installed ubuntu just this past weekend. so im probably too newb to answer your question.
In Ubuntu at the top, go to System > Administration > Hardware Drivers
There should be a driver listed for your wireless Internet. Select it and then click on activate at the bottom. You may need to restart for it to work.