I'll just write you a guide for installing - a first time install is a little nerve-wracking, but after that time (especially if you do it a lot, like me) it becomes like second nature. If you spend some time looking around with it, you'll probably learn a bit about partitions and how installations and booting works. You'll also learn about your BIOS or UEFI, which is a pretty key step in interacting with your computer at a lower level.
Manjaro is a great place to start! It's pretty easy to install, so there's that. We might as well just jump in.
First things first - you'll need some prep. In Windows, in File Explorer, right click on Computer, select Manage, and when the new window pops up, select Disk Management from the menu on the left. Here you'll see your two HDDs, with a convenient graphic on the bottom that shows your partitions as Windows sees them. Your partitions, since they're already configured with Windows, are probably already MBR, so you won't have to worry about that. You said you have files on that HDD, right? Here you can right click on the HDD and "shrink" the volume to how much space you think you'll need, and leave the rest unallocated for Linux. You should be able to get away with as little as 40~ GB for Linux, but I would recommend 128GB so that you have a lot of room for applications and whatnot as you see fit.
Now that that's over with, I'll assume you have now downloaded Manjaro XFCE from the website. If you have a flash drive, plug it in and format it in Windows (easy as right clicking and formatting it as fat32). Get an image installer like UNetbootin and follow its instructions to get the distro onto your USB drive.
Now that that's done, once you're ready, restart your computer. If you haven't already, go into your BIOS and enable booting from USB, and make sure that your flash drive is on top of the boot order. That's probably a lot to take in at once, but as long as you can get into your BIOS or UEFI from the start, you are in a good position. It's usually a good idea to look around at all of your BIOS options anyways to get an idea of what you can do with it, and set a password on it if you're paranoid.
Finally you can restart your computer and boot into your USB. Select 'Default' or 'Start Manjaro' (not sure what options UNetbootin would have here; I usually compile the image directly in Linux), and Manjaro will spend some time starting some of its services and will bring you to the desktop, if nothing goes wrong. If you want to jump straight into the installation, on the right side of the window that comes up are the buttons "Install Manjaro" and "Install Manjaro (cli)" (they're also on the desktop if you want to play around a little bit first). Click on the first one for an easy graphical install, and follow the instructions until it asks for where it installs, where I will get back to you below.
Now, it should ask whether you want to erase everything on your HDD and install Manjaro, or install your partitions manually. Select the latter, and it will come up with a menu where you can see your partitions for your HDD. Chances are, it's on your Windows drive (probably /dev/sda), so select your other drive instead (probably /dev/sdb). Make a partition for your swap (8 or 16GB will do fine) and partition the rest (minus about 300MB) for your root partition. ('/' is your mount point, if you're wondering.) Make sure at the bottom you change where GRUB installs to the second drive as well. That will fill some of the unallocated space you have at the end of your drive. Continue, fill out any other questions it has for you, finish the installation, and restart your computer. (Don't forget to take your flash drive out. :P)
You might have to go into BIOS one more time to make sure that your new Manjaro installation is first in your boot order. Beyond that, you should get into GRUB with the options 'Start Manjaro [...] Windows (loader).' Congratulations; you are now running a native Linux distribution. :D