Which bootloader are you using? Some popular ones (like GRUB) can use the firmware to set up a high-resolution video mode with a framebuffer where you can draw bitmap text and graphics.
If you wrote your own bootloader, you can have your bootloader do it too. Since you mentioned VGA text mode, I'm assuming you're using legacy BIOS, so you'd want to
set the video mode using VBE. For UEFI,
set the video mode using GOP before calling ExitBootServices().
Changing the video mode while your OS is running is a bit more difficult. For that, you need to write a driver for your display adapter.