[ubuntu] How to manage partitions when having installed Ubuntu besides first installed Windows

Hi! I am new to Ubuntu (installed it only today!), but have since many years gotten more and more irritated over Windows attempts to intrude into my whereabouts, and the risk of getting a share of all viruses etc out there on Windows systems. So, now that I finally bought a new ASUS miniPC with some 500+ Gb memory, I wanted to install and use Linux instead of the preinstalled Windows 10 that came with the buy. My colleague pointed me to Ubuntu 14.04 LTS. It was downloaded to a CD, and used for the installation, as below described.

I did not want to remove the Windows 10, just install Ubuntu beside it, to choose which OS to use whenever I start the computer up.

Now my problems and questions:

1. When I started the computer first time, of course the preinstalled Windows 10 was first launched and prepared. It then divided the memory into two partitions C: and D:, with somewhere around 130Gb in C: and more or less the rest in D:. Then, when I was to install Ubuntu, the installation went seemingly well.

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a) But, Ubuntu seems to have used Only C: to install in, same partition as Windows 10 was installed in, leaving the D: unreachable for Ubuntu (?). I was under the impression that Ubuntu would give me the possibility to handle/manage the total memory independent of any other OS installation. How can I do that?

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b) Further, out of those 130 Gb in C:, Ubuntu now divided that into about 45Gb for itself (a new partition?), and the rest (about 84Gb, a second partition?) for 'Files'. It seems that my nice and big memory now have been reduced to 'only' about 30-40 Gb, from the initial 500+Gb. How can I change this?

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c) I like order. I would prefer (and this is what I have done all the in Windows) to 'partion' and store OS files (for Windows as well as for Ubuntu) into their own folders or partitions, to keep them separated from all other installations and data files, etc. How can I do that?

Thanks for your suggestions!