If you want to use GRUB and UEFI, my recommendation is follow the headings up until the end of Installation
https://wiki.archlinux.org/index.php/GRUB#UEFI_systems
Then you just need to generate the config
https://wiki.archlinux.org/index.php/GRUB#Generate_the_main_...
The partition you're referring to is explained here
https://wiki.archlinux.org/index.php/EFI_System_Partition