Assuming you bought Starbound on Steam, you should be able to play it right now by going to your Library tab on Steam and clicking play. A display screen will pop up with 4 buttons on the bottom. You can click "Launch Starbound (OpenGL)". The version without "(OpenGL)" is really slow for slower computers.
This being a pre-ordered game, but it is also an early access game, meaning you are allowed to play the game while it is being developed. Bugs and data wipes are to be expected.