Before you install
Before installing MU, it is very important to do a few things. The utmost important one is to read the readme.txt file. The installation procedure is not the same as the famous 5 minute WordPress install; it is quite different. Become familiar with the contents of this file. If at any point you have questions, the best people to ask would be your web host. It is likely if you ask pre-install questions in the forum, most volunteers will tell you to ask your host anyway.
Another step is to decide how you want your blog address to use. If you want subdirectories like http://domain.tld/username/, then you do not need to do anything. Subdomains, addresses in the form of http;//username.domain.tld, are generated virtually and as such, virtual hosts (Vhosts or wildcard domains) have to be enabled on your hosting account. Talk to your webhost if you cannot do this or if you can’t follow the included instructions.
You also need to figure out your intended audience. This will help you customize your site to stand out from the rest and make better decisions regarding what you and your users need for a successfull blogging experience for them, and less work and frustrations for you.