To be honest, Fire Emblem is such a popular series of games that a lot of people have already worked on, so much so you can google something like "Fire Emblem GBA editor" and actually get text and graphics tools in the first couple hits. How well they work is yet to be seen, but it really, really helps when you aren't alone on something.
RHDN is a great general site for hacking, but you'll also want to look at dedicated communities. GBA is a heavily hacked system and you'll find most of the popular games have their own hacking communities. In all likelyhood you'll be able to find codecs and ROM maps, plus utilities to streamline it all.
It also isn't the nightmare of other systems, with peculiar hardware configurations, etc. If you have to pick a FE to play with, GBA is probably the easiest choice.
It will still take a lot of your own work, and don't think you'll be able to get away without learning something of the system itself, but with a bit of initiative you should be able to get started rather easily.