How to install wpmu in godaddy hosting account?

I am brand new to wpmu. Just started today at this site and I would like to know how to install wpmu into a godaddy hosting account? Are they the best hosting for this? should wpmu be the primary domain in the hosting account or can it be a subdomain? I do know that I cannot overwrite the blog that I currently have and would like to replace wpmu with, but is there a way to do this? I have also download a theme and would like to know how I can install it as I am not ver familiar with wpmu and theme instalation is clearly different from wp. I have search the forum but did not come accross these direct answers. So I am asking them. Thanks for your help