Welcome!
For starting out a romhack, especially your first one, I’d highly suggest trying out FEBuilder: FE_Builder_GBA -- If you have any questions, attach report7z
Mess around, the more you use Builder the more you’ll know!
For what you’re doing, you can definitely do it pretty easily in builder.
Send a report7z (or description of your problem if it’s very basic) to #febuilder_help on the discord or… somewhere here if you’re truly stuck.