I can't boot from USB in grub

I'm new to using Linux and I tried googling an answer but I didn't understand the instructions. I installed Lubuntu but I must've messed up somewhere because my computer goes directly to GRUB. After pressing tab and trying a bunch of commands, I was finally about to load the BIOS thing in order to have it boot USB first. For some reason, I can't boot the USB and nothing happens when I click F12 and choose SanDisk from the boot menu. The ISO is for CloudReady if that helps. I read online about booting from within GRUB but it tells me I have to do something with the kernel first. I think the file is startup.nsh since that's the only file I could find while checking each hd. It would be much appreciated if someone could help me with easy instructions since I've searched for hours and tried everything I could find but I didn't understand how to do it properly. Sorry if I wasn't clear so I'll try to list what I did. - I have an old Gateway computer sx2110g with Windows 10 installed. Windows was slow so I wanted to try a version of Linux. - I tried installing Lubuntu but I must've messed up because my computer only boots in GRUB. - After looking online i found that typing fwsetup on Grub let's me restart into the BIOS. - I wanted to install a different OS so I set "removable device" to boot first. - When I restart it only says "_boot loader not found_" then immediately goes to Grub. - I tried f12 to manually choose the USB drive but the same thing happens and it immediately goes to Grub. My question is that if there's a way to boot a USB from within Grub since that's the only thing that's opening for me on the computer. I tried these instructions: https://szymonkrajewski.pl/how-to-boot-system-from-usb-using-grub/ I was able to figure out which drive is the USB by typing "ls" and searching each drive but I don't know how to boot startup.nsh.