Hello.
To do this, I recommend using
GBA Intro Manager, which has an option to change the spawn of the player at the very start of the game.
Just to remind you how maps work, if you open a map editor of choice (then open a ROM you want to hack, and then the tab 'From Header', the first option, it shows a list of numbered tabs, that's the Map Bank, or the 'Bank' in GBA Intro Manager, if you click onto one of the banks, it will direct you to overworld maps of the game, the second number listed on them is what the 'Map' option corresponds to in the GBA Intro Editor, and finally, X and Y are the coordinates that you would like to place your spawn on.
In short, choose the map bank, and the map you would like to spawn your character on, and then find the exact coordinates of X and Y (bottom left of the screen usually for map editors). Insert the map bank, map number, and coordinates into GBA Intro Manager, and then save. It should then direct your character to where you assigned them.
Hope it helps. :)