I'd like to point out that you can see all 150 and get the National Pokédex without beating the game, as long as you trade for the Pokémon you won't be able to see yourself (specifically Dialga/Palkia, whichever you don't catch yourself). It is just standard that most people get the National Pokédex by seeing the Elite 4 and Champion's Pokémon, and seeing Dialga/Palkia from the Celestic elder.
Anyway, to transfer from a GBA game to a DS game, you need the National Pokédex. Once you get that, the Pal Park (at the end of Route 221) becomes available, which allows you to migrate 6 Pokémon daily from your GBA game into the DS game. This is a one-way trip, though, so be aware of that.
And this only works with GBA games. The original Red, Blue, and Yellow versions, as well as Gold, Silver, and Crystal versions will not work.