This will only work if you have 2 GBA's and 2 Games. (Or you use a friends) Start the game and get the starter pokemon you want. Then trade it to your other game and restart your game. Then if you got the fire type, get water this time, if you got water, get grass, if you got grass, get water. Now trade back the starter pokemon you got earlier. Now your rival will have the opposite type pokemon as you so you will have an advantage and hey, you get to starter pokemon.