Welcome to LQ!!
The standard routine is this:
- Backup important data
- De-fragment the Windows partition(s), if necesary
- Resize existing partition(s) to make room for Linux. (This can be done from something like GParted live CD, and linxu live CD, and (I think) from some versions of Windows.) Create at least 10GB of empty space---preferably more.
- Boot up from the Linux CD and follow instructions---tell the installer to use the empty space you created.
Almost all modern installers will detect Windows and automatically set up the bootloader.