I'm assuming you already downloaded the ubuntu ISO file and burned it to a DVD and did a trial run to see if it works well on your pc and you like it (if not, speak up as you need to do that correctly).
When you boot off of the Ubuntu DVD, it will ask you if you want to install so choose that. If you are new to linux and all this, my suggestion would be to proceed as follows: You should get to a screen with some options and the first one will say "install ubuntu alongside windows 7" (if it doesn't say anything like that I would cancel and come back here!). If you choose that and proceed it will automatically install it alongside your windows 7. First, it will show you what it decided size wise to divide your hard drive (you can slide the sizes back and forth to fine tune it.
Here is a link for more info https://help.ubuntu.com/community/Ho...dowsPartitions
Again, this my advice for a newbie (I did it a few months ago with XP and Xubuntu, which uses the same installer, and glad I did) to jump in and get going. I am going to get a lot of flak for this because everyone else will most likely tell you to pick the manual option and create a home partition and so on. And that is certainly the absolutely best and greatest way to do it. But you are more likely to screw it up that way so I say it's more important to just get on with it and learn how to use the ubuntu. Later, you can always go back and do it differently (not ideal, but by then you will at least know what you are doing).