You should be able to get into the UEFI firmware setup utility by pressing a hotkey at boot time ─ the documentation of the motherboard (or of the machine) should tell you what key to press, and it may even appear on the screen during the POST routine.
Then, make sure that Secure Boot is disabled, that the machine boots in UEFI mode only ─ no CSM ─ and that the machine can boot from a USB stick if present. And then you boot up from the Manjaro stick and you wipe the Wintendo™ partition.
When creating a partition table, create it as GPT, and create a
vfat (FAT32) partition of about 512 MiB, with
/boot/efi as the mountpoint and marked with the
boot flag. And then you can create your other partitions. I recommend a separate partition for
Depending on the amount of RAM in the machine, you may also want to create a swap partition, of at least the size of your RAM if you plan on hibernating the machine.
Personally, I’ve got 16 GiB of RAM in this machine and I’ve disabled my swap partition, because this is a desktop machine, I don’t hibernate it, and with 16 GiB I’ve got enough memory for what I do with my computer. But your mileage may vary.