As mentioned that is not as much jumping in at the deep end but standing on a cliff and trying to do an Olympic level dive off it and learning to swim at the bottom.
Two player hacks take several approaches, assuming it was not a feature the devs left on the cartridge but gated off for some reason and you can just unlock that.
1) Standard NES Mario style where you pass the controller on death/level end.
2) Games where you are followed by a NPC that you can then kick control to, or an AI you can take control from.
3) Actual straight adding a second player where there was none before, and if you are doing that on a GBA that also means tackling link cable protocol. This is by far and away the hardest approach but also the one with the best results.
1 and 2 sometimes meet in the middle for things like turn based strategy. There is also a share the controller duties (think doing something like mario kart double dash, or maybe a fighting game where you can tag in another character).
For this you would probably have 1) and 3) as options.
1) Might not be so bad. I would probably take the second save slot and give it to the other player and alternate between them every death/level end. If you are going to be playing this on an emulator use a savestate and just pass the controls over, maybe also have something to add a life as a cheat.
3) Nightmare time, especially if you are doing a link play hack. Not to mention you would presumably also have to redo the levels to change pickups, or maybe do something like megadrive Sonic games with Tails where it is essentially an AI helper that can come and go at will/returns if you go too far.
People starting out usually learn to edit some text, tweak some graphics within existing parameters (change some pixels around but keep it the same size, change some colours), maybe figure out level editing and as it it the GBA you will probably have an easier time editing audio than some older consoles (many GBA games use the "sappy" sound format rather than the "everything is custom" that older stuff tends to see).
I do have a guide to GBA ROM hackinghttp://www.romhacking.net/forum/index.php/topic,14708.0.html
You will also want a programming tutorial on the GBA for which http://www.coranac.com/tonc/text/asm.htm
(that is the assembly side of things, might want to go from the start of it).
and a guide to then GBA hardwarehttp://problemkaputt.de/gbatek.htm