Aftermarket stereo installs are super easy and a great first project for an emerging DIY'er....but first...you have an LS which means you most likely have the Bose 2100 which utilizes a central amplifier located behind the rear seat. To use an aftermarket stereo that operates at 4 ohms impedance, you will either have to bypass the amp and get 4 ohm speakers, or get a Line Level Converter to use the 4 ohms stereo with the Bose Amp and speakers.
I know it sounds complicated but its really not....
Scosche SLC4 Four-channel line output converter at Crutchfield.com
This can be had at Wal*Mart for about $20.