Photo Credit: Courtesy of 3DStockPhoto ( image)
Your nozzle is too far from the build plate. You should make some changes to bring the nozzle closer to the build plate for the first layer. There are a number of possibilities for doing this:
Adjust the Z-axis endstop
Loosen the bed leveling screws
Add an offset in G-code
This will squish the plastic down more, enabling it to stick to the build plate.
With solution (3), you have to be careful that you don't crash into the endstop. This solution works best if your printer homes towards max (and not min) or if you have non-mechanical (hall or optical) endstops.