More detail might help.
It's actually easier to install Linux to a blank hard drive than it is to install it to one that already has stuff on it, because you do not have to delete or resize any partitions.
The short version is to download an *.iso of the distro you want to install and burn it to a CD/DVD. Be sure to use the "burn image" tool in your burning program, which will take the *.iso and recreate the install media; don't fall into the trap I did once of just copying the *.iso to the disc; all that gives you is a copy of the *.iso file, not a functioning install disc.
Then, on the computer to which you wish to install Linux, boot to the disc (you may need to enter the BIOS or "setup" to tell the computer to boot from the CD/DVD drive before it tries to boot from the hard drive). Some computers, during initial boot, give you a dialog to "Press F[something] to choose a boot device." Others, during initial boot, give you a dialog to "Press F[something] to enter setup"; in that case, "setup" means the BIOS settings.
Once you've booted to the disk, follow the prompts to partition and format the hard drive and install the OS.
The website for the distro you wish to install should provide instructions for the install process for that particular distro.