Select Adjustments tool (A). Turn on "Levels" and drag the handles to increase the contrast.
Turn on Black and white.
In Format menu, select Convert into Pixels.
Invert colors (type Cmd+I or open Format menu) so that the background is black.
Select Effects tool (F). Click Add Effect → Other → Mask to Alpha.
Ta-da! You may also want to add a color (Add effect → Fill → Color).