If you're ripping it from a copyrighted dvd you can't use disk utility because technically it's against the law. Otherwise yeah you just open disk utility, choose the disk, and select Image from Disk in one of the menus. That'll create a .dmg, if you need iso you have to make it uncompressed master cd/dvd in the format option, at which point you can change back and forth between .dmg and .iso freely(it might be called .cdr too, either way you can just change it to .iso)