I apologize for being long-winded, but I figure it's best to give all the details rather than be too vague. My basic problem is this: I just got a new CyberPowerPC desktop with a 240GB SSD and 1TB HDD, and Windows 10 pre-installed. I want to dual boot Windows with Ubuntu Studio (version 19.10), as well as have a partition on the HDD accessible to both. I thought I installed Ubuntu properly, but the computer boots straight into Windows every time rather than loading GRUB. I think my issue is with the way I partitioned my disks, but I think it's best to ask and be sure before I screw anything else up. I've looked for other answers but none seem to match my situation closely enough for me to use.
Now, for the full story: I made the live USB for Ubuntu Studio, and resized the partition on my HDD to make room for it. At this point, I was thinking that most of my programs/games will be installed on Windows, so I would let it have the space on the SSD and give Ubuntu 300GB on the HDD.
However, when I went to boot from it, nothing happened and Windows started up as usual. I discovered this was because of the secure boot setting on my computer(I had no idea newer computers worked this way), and went about getting around that. I went into the computer's UEFI settings, and didn't see anything that said "turn off secure boot", so not wanting to screw anything up I went back to Windows and used Shift+Restart to open the boot settings menu and start from the USB that way. Now, when I did this, there were two USB stick options: one simply labeled "USB Stick" and the other labeled "UEFI: SanDisk, Partition 1". I had chosen the plain "USB Stick" option rather than the other, so this could be one thing I did wrong.
In any case, Ubuntu Studio's installer opened, and aside from the fact that it didn't recognize my Windows installation("No operating systems have been detected"), everything appeared as normal. When I went to install it to the 300GB partition I had created, I got a message asking for a bootloader partition marked as "Reserved BIOS Area". By my understanding this refers to GRUB, so I went back to Windows and freed up 1GB for it on the SSD, figuring that would be enough space(and by now figuring the SSD was first in boot device order). Going back to the Ubuntu installer, everything seemed to proceed normally and finished saying the installation was successful. But when I reboot the computer, it boots straight into Windows without ever loading GRUB. I tried changing the boot device away from "Windows Boot Manager", but that just got me a lovely black screen.
My guess at this point is that I should have just partitioned the SSD in half and installed Ubuntu there, leaving the HDD as storage space, but I'm also not sure if the way I booted into the USB stick is a problem. Is there anything I did wrong that I'm not seeing? I want to be sure I'm doing it correctly before I install again.
Thank you for reading through all this.