You can start transferring off Ruby, Sapphire, Emerald, FireRed, and Leaf Green, once you've seen all 150 Pokemon from Diamond/Pearl. Fly to Sandgem town and go to the professor's lab. Professor Oak will be there. He will have a speech then he'll upgrade your Pokedex to a national. Now you can successfully migrate Pokemon from your GBA cartridges. You can migrate up to 30 Pokemon daily but 6 at a time. To start Migrating you'll need to turn off your game and restart it until the starting menu comes up. Then click on Migrate Pokemon from Game. But you must have the GBA game inside the GBA cartridge of your Nintendo DS (Slot 2). Once you've clicked onto Migrate your pokemon, Just simply follow the steps on there and select your 6 Pokemon. If your Pokemon knows a HM move, I suggest deleting it before transferring because you will not be able to transfer it.
Now once thats all done, transfer your Pokemon and fly to Route 221. Your Pokemon will be waiting to be caught via Pal Park. There are certain area's for each Pokemon, there is Wood's, Pond, Grass field, Mountain & Sea Area.
Legends & Rares are harder to find than basic Pokemon like Catipie.
Once you've entered the actual Pal Park Area (shown in the image above) You get 6 Pal Park Balls, They have a 100% catch rate, like a Master Ball. You will be timed and once you've caught the Pokemon, you'll be ranked on how long it took for you , Pokemon Type, and over all your catching show. Then the man will give you your score & you will receive a berry. Then he will ask you if you want to deposit the Pokemon into the PC, say yes. Now you've successfully migrated Pokemon over. The next time you'll be able to Migrate over on that game will be in 24 hours.