Welcome to LQ!!
Do some searches here and on Google---this is a very common question....
The basic procedure:
- Back up your data
- De-frag the windows partition
- Re-size the windows partition to create empty space for Linux
- Install Linux---most installers will automatically detect the Windows system and set up the bootloader.
Depending on the age of the Windows installation, how much data you have, etc., I would consider doing a clean install of Windows---giving it only 15-20GB. Give Linux another 10-15, and use the rest of the drive for shared data.
Did I mention backup?? Ah...yes