What you want sounds more like savestate hacking than romhacking. Play the game and write down a stat you want to change. Save your game and search through the saved file for that number in hex form. You'll most likely find lots of places where it occurs. Write down all places and play the game some more. When the stat changes write it down again and save. Now look at the places you wrote down last time and look if one of them matches the new number. If several are still the same repeat the procedure as long as needed until only one value remains. That's the offset you need to change.

THANK YOU!!!  That help SO much.  I was really confused about all this stuff, and those two posts helped me immensly. 
When it's compressed, how do you tell and how do you fix that?

Down the bottom, check out the guide by yours truly: http://www.gamefaqs.com/portable/gbadvance/game/561356.html.

You can use VBA's built in cheat searcher to help with this sort of thing, there's a series of guides at http://cheatandwin.com/forums/showthread.php?t=127 that explains it.

The addresses in the guide, and the method of the cheat searcher give you RAM addresses.

If you want to edit saves directly (and to be clear here, I'm talking about SRAM/'battery' files - with a *.sav extension, VBA savestates are gzip compressesed), you usually find that the data structures present in RAM are the same in SRAM, so by searching for contiguous blocks of data you can find them quickly.

Alternatively, there are programs out there that help with the comparison of save files that Kaioshin described - I'll see if I can find some links for you.