Its really easy to do that. when your installing whichever distro you choose, when you get to the part about where to install, there is most likly going to be an option "install side by side with my current OS". Some distros don't have this yet, most do. If its not an option, then select that you want to do the partitions yourself and set it up like this:
1. shrink the windows partition by the size you want your linux part to be. leave all other partitions the same.
2. in the new unused space create a new partition, in the popup menu select the filetype as "SWAP". make this 3-4gb. swap is the virutal ram on the hard drive linux uses for overflow.
3. in the rest of the unused space create a new partition, choose EXT4 as the filetype (or EXT3 if its not there). make the mount point just "/" and make it as big as possible, this is the actual partition linux will use.
now just hit continue, it will do everything from here. when you turn on your computer you will get a menu that you navigate with the arrow keys that lets you switch between linux and win7. other partitions (like recovery partitions) will also show up in the menu, so don't be afraid if theres more then those 2.
Linux is totaly awsome, so much better then windows. even though i greatly prefer linux, i keep windows on for solidworks and labview and visual studio. there are c++ compilers in linux, so you don't need to worry about that.