I'm very sorry if I confused you. Your first post made mention that you wanted to install Qubes OS on a different partition on your HDD. I did not see that initially. I saw your edited post and it made no mention of Qubes. Just disregard that part and I will try to answer your questions for you.
Its the other way around. You first install Qubes OS (the host). Then you would install your Whonix VMs (the guest).
You can find the Whonix page in the Qubes documentation here. If you use the current stable release which is Qubes 3.2 , you don't have to worry about downloading Whonix. It comes installed by default
To recap, you install Qubes OS ( the host ) first. If for some reason Qubes did not have Whonix installed by default you would then install your Whonix VMs.(your guest VMs)
I would recommend you read through the Qubes documentation. A good starting out point is The Qubes OS intro. After you have read that go to the Qubes video tours. It was made for Qubes 3.1 but the information still applies and the narrator does a great job of explaining about all things Qubes.
After you have finished with that go over the rest of the documentation just to familiarize yourself with the OS. If you have any more Qubes related questions sign up to the Qubes mailing list. I posted the link in my previous post.
Since you only have one HDD I would install Qubes on a USB drive at first just to make sure you don't have any major hardware compatibility issues. If you do end up having any problems the mailing list is a good place to start searching for answers. Just so there is no confusion the Google Groups Qubes users forum link (from my last post) is actually were all the questions and answers from the Qubes mailing list go. If you do end up having problems there is a good chance someone else has previously had the same issue and has found a solution. Once again make sure you look for your answer here on Google Groups before you post a question on the mailing list.