Can't transfer to black and white from gba games.
Insert leaf green and either diamond or soulsilver.
Make sure that Pikachu is in your PC
You cant transfer just one, you need to transfer six, so make sure six pokemon you want to transfer are in your PC
Next, open the ds game, get to the main menu, but instead of Pressing Continue, scroll down to Transfer from Leaf Green. Go through the prompts and select the six Pokemon you want to transfer.
Then head to Pal Park (in SoulSilver in Fuchsia City, and in Diamond at the east end of Route 221), enter Pal Park, speak to the person at the desk.
Then you just have to catch 'em all in the park, make sure you take a pokemon that knows surf if you transfer across a Water-Type pokemon as well.
Hope this helps :)