Adobe have said they are working on one, but not how soon it will be ready.
Aperture won't stop working suddenly, so why not continue to manage all pictures taken before today (or some other date) in Aperture, and have all new pictures in Lightroom? That reduces the risk of confusion. When the migration tool is ready, you'll have more experience of LR and be better able to resolve any problems that may occur.
My understanding is aperture will be there 'forever' but it not get anymore updates
John made a good point getting used to LR before adding heaps of files. IMO a good books beats videos when learning the basic of LR