It’s been covered in many posts (that’s why this is an FAQ after all), and this one is a reasonable and concise representation of them.
Well, as it says in the activation email you got when you signed up to the forum , start low. If you really have no idea where to start, then go with the lowest scaling on offer.
The WOD will come around again and you can always move up then.
When to move up a level?
When you’re doing the WOD about as fast as other top performers.
Good sources of info for times for WODs are the Brand X underdawg list http://www.crossfitbrandx.com/index.php/forums/viewthread/1100/ and the comments on the mainsite WODs http://www.crossfit.com/
There are no hard and fast rules on this aspect so if you’ve had a look at those places and you’re not sure, your best bet is to ask when you post your results on the WOD forum.