You could mess with GParted right out of the gate and do it, but I think there may be a better way.
The better way to do it, IMO, is this:
When you're confident enough in using Ubuntu, just reinstall it from the LiveCD. There will be a screen in the install routine that will allow you to wipe out all other OS's and only install Ubuntu . . . choose that option if you want to get rid of Windows.
Plus, in the install routine you should have an option to mount the /home directory on a different partition . . . take it!!!
Now, as far as allocations, the install routine will take your whole HDD space if you choose that "wipe out all other OS's" option.
But whs is right, 40GB is plenty for Ubuntu, actually 20 or 30 is OK.
So, when you get done installing, and BEFORE you install other software (which will install itself to your /home directory, not the root), run GParted from a LiveCD (because GParted from your installation WON'T manipulate MOUNTED partitions . . . you have to do it from a LiveCD), and allocate maybe a 10GB partition for your root (which will already be there, so you'll just "shrink" that partiton . . . likely sda1).
Then make an extended partition (likely sda2), and this will include your "/home". Then shrink your ''/home" down to about 20 or 30GB, 'cause this is the file system that will grow (it will hold all your data . . . though you should store your valuable data on removable media, like USB sticks . . . /home will just contain your user app file configurations in what they call "dot files".) Your "/home" directory will grow as you install new software.
Now the reason I say to do all this "BEFORE" you install other software . . ." is that if this GParted manipulation goes south (and they do some times, especially if you try shrinking operations), all you'll lose is the original LiveCD installation . . . and you can just do it over again from scratch.
Tedious? Maybe. Safe? Yes.
Now partitioning operations can be complicated, so I recommend you read this: https://help.ubuntu.com/community/HowtoPartition and completely understand it BEFORE you start into partitioning operations.
I also recommend reading this BEFORE you start: http://members.iinet.net.au/~herman546/p22.html Now I realize this is for "dual boot" and that's NOT what you want, nevertheless it's a good guide for partitioning.
All you really have to do is make sure you don't do ANY Ubuntu software downloading until you get the partitioning the way that it's most efficient FIRST, then you can go ahead and start using it normally. This way, if things go south on you or you don't like how you have things arranged, all you will mess up will be the installation and, as I said, you can start over again 'till you get it right.
In the alternative, you CAN try GParted right out of the gate, and if worse comes to worse you can do the install routine from the LiveCD. That's the beauty of Linux: If one way doesn't work, you have alternatives.
Some recommend using the install routine partitioning, while some recommend prepping with GParted first. There's advantages and disadvantages to both. Try it either way if you like, but if you have valuable data, make sure and back it up to removable media first.
You can also back up any of your "/home" dot files if you already have some app configs in Ubuntu that you want to keep, but be aware that some of these "dot" files are pretty big . . . you may need some large capacity USB sticks for them. Then once you install from the LiveCD and get things the way you want them, you can simply copy and paste those dot files into your new home directory. Then all you have to do is reinstall the apps you use.
(To get a list of the apps you currently have on your Ubuntu, type this in the terminal: "dpkg --get-selections > installed-software" - WITHOUT the quotes - the list will show up in your home directory as a file named "installed software")