Visit the Home Center and buy some Concrete Cleaner. Follow the Instructions and you're on your way. Make sure you have the 'tools' too to do the job (like a small brush, and such, depending on how the concrete is to be cleaned.) Be aware that you MAY need to eventually clean a much LARGER area because you may find that the 'spot' is not a 'clean spot', and sticks out as bad as the dirty one did! Soooo, clean the spot and by more Cleaner in case you need to clean a LARGER area as well.