blogging help please

Hi
I have a question please. I want to set up a blog to my site from wordpress so the blog url is /blog after my domain name. I already have a blog like this that someone very kindly set up for me. I have wordpress mu as suggested by a forum member and have uploaded this to my root server and to my webspace.
But this is where I get confused forgive my lack of knowledge, but I am still learning my way around all this technical stuff !!
How do I now access the files and actually create a blog? with my other blog I have a nice little blue box I simply put my user name plus password in and hey presto I am in wordpress and able to blog away happily.
I am confused can anyone explain in very simple terms what I do now these files are uploaded, I have not written a blog for my new site yet I don’t know how to get into it, help please somebody.

Janine