You're not precise enough in your question.
When you're looking for a solution in networking do.
1. Ping an address ip on the internet
2. Check if name resolution works using nslookup or dig
3. Then try to use your browser
if you wanna use your linux box as a gateway read the netfilter's documentation regarding nat and masquerade.
The two important steps are
- activate ip_forward
- write a rule to masq your subnet
I can't browse the internet means nothing to me, it could be you ethernet cable, your network connection to the internet, a dns problem, the website you try to reach is not available and thousand more issues.
For your usb modem try to look for information on a search engine, is it supported by linux ?
Then search documentation on how to install it.