If you type Modelsim in the search engine on altera.com, the first link brings you to a nice collection of the documentation. If you have the time, it's good to read through this, but an excellent way to get started is click on the Modelsim Tcl Scripting Examples. This has Tcl scripts where you just fill in the the path to your Quartus install and type of simulation, and it reads in the correct libraries. It's an excellent starting point with exact commands to get you going.
http://www.altera.com/support/examples/tcl/tcl-modelsim.html