Are you wanting to be blonde all over?
I would recommend you bleach your natural hair BEFORE ordering a blonde piece because you won't really know what shade of blonde your natural hair will be until its done.

You also have the regrowth problem where your dark roots will start to come through quite quickly on your natural hair, but obviously not on the piece.

To keep it perfectly blonde all over will require a lot of touch ups and maintenance.
My personal recommendation would be to go for blonde streaks and highlights.
Then the darker regrowth won't be such an obvious issue.
http://youtu.be/iUVCSDP2MT4