The majority of Unix systems are "nice" to you. When you attempt to install them, in the presence of another operating system, they'll say (each in his own way):

"Hey, dude. There is another OS in town. He is sitting on that corner (then gives you partition/slice information, hard drive mapping, and other information) Would you like me to 1- kick him out (delete) or -2- beat'em up a little bit (resize partition/slice) or -3- move in the empty side of town (install on the available free space)

Then asks if you want to 1- let the other OS boot -2- Unix boot manager kicks in and gives you a list of available OS's -3- just boot Unix and forget the other guy.

To get a copy of Unix/Linux, check Rules & FAQ

Where can i get a copy of Unix or Linux?

Good luck