I am assuming you just have a single hard-drive on your machine.
You need to identify carefully(sometimes there are hidden partitons).
First information you need is to find if your partitions on the drive. And what are their contents.
Find if they are primary partions or Logical ones.(There can be only 4 partitions on a drive. Your 4 may be primary while a 5th be logical)
You will need to free at least one primary partition.(Chose carefully), By free i mean destroy one partition to get free space.
You may also remove all the other recovery drives but that is upto you.
This can be done from the manager in windows or using third party software (easily found by google search)
Burn a Ubuntu latest version .iso file to a DVD. You can find that here.
Insert the disk and boot from the CD and perform installation of OS.
If you don't want to sacrifice the recovery drives. A wubi installation from the Ubuntu Disc is also a option which will install ubuntu inside your windows.
Use a virtual machine software like (Virtual Box) to run the Ubuntu on windows(wubi is a better option though)
IF you just want grub from Ubuntu to recognize your Windows part ion for a dual boot
follow this link:
Grub does not show a Windows 8 option after dual boot