At work we are on a network with shared computers.I'm not able to connect to the internet from the network,I can only connect to their intranet.How may I connect to the internet,I don;t have a username or password--do I absoluely need one?

To put it simple, you cannot until a network admin gives you access.

There are several cases possible:
1) There is no internet access.
2) The pc access are filtered by ip or MAC adresses, and only some pc have access open for the web
3) You need a login/password with privilege to unlock your ip/mac adresse to access the web

IMHO, if you cannot access the web, it's probably because your enterprise ruled that out.
Keep in mind that everything you do on a network can be traced to the computer / account which did it.
And on a lan, there is much less noise coming from the outside, meaning that a lot of repeated attempts will certainly catch a serious sysadmin attention.
And you would'nt be the first to get fired because you've attempted to get past the security put in place by your enterprise.

Really, I'd think about it twice, if I where you.