If you run multiple WordPress sites, you should learn about WordPress networks.
At first glance, it might seem simpler to just install a new copy of WordPress every time you want a new site. But think about updates. Every time a WordPress update comes along, you'll need to upgrade every site separately.
And how about plugins? They need upgrades too. How do you track which sites have which plugins? Once you have more than 2 or 3 plugins, this could turn into a nightmare.
Consider a WordPress network. It's a much more elegant solution.
The first step is easy: install a normal WordPress site. Or, you can turn an existing WordPress site into a network.
Next, you'll need to do some administrator magic. It may seem a bit arcane, but with these setup instructions, you should have your new WordPress network up in an hour or less.
Once you've got a WordPress network, it's important to understand how the administrator screens will change. Now that you're managing multiple sites, you'll go into different "modes." Your options will change, depending on which site you're managing.
One major change is how a network handles plugins and themes. This is good: the new behavior is a major reason to use a network! But you still need to understand how a WordPress network handles plugins and themes, otherwise you'll get frustrated.
Once you're comfortable with your new network, it's time to take the obvious next step: create a new site! A network makes (almost) everything about creating a new site extremely fast and easy.