I want to begin photography as an hobby and I thought to start with the right foot and take my photos in raw: is a good choice for a beginner? I have a new MacBook with Retina display, so a powerful laptop with a very good screen. What is the best program to processing raw images? Lightroom? Or Aperture (by Apple) is a good choice for a beginner (it is not so expansive!)? It is a professional program?
I use Lightroom, but that is because I like the integration with other adobe products. It is really easy to open a photo in Lightroom in Photoshop from light room and then send the updated file back. I have considered switching to Aperture, but I felt like the time to move my photo library and learn a new system wasn't worth it.
That being said, from what I've read about Aperture it is a high quality application. I think the decision between the two will come down to very minor personal preferences similar to choosing Nikon or Cannon. It will come down to how you plan to use either app.
Adobe is releasing a retina update for Lightroom 4 to improve its use on the MBP Retina.
To add to what CyberKnoy08 said,
If yo ever used any other Adobe program, I suggest you to get the Lightroom, ofc their price is much different but you'll be in a similar environment.
Both programs will do the job and both are used by professionals and also beginners.
Aperture is a great program, used by many pros. And it supports new cameras fairly quickly. I expect that Apple will (or may have already) made the port for the Retina display, after all its an Apple MacBook Pro.
Aperture is really inexpensive when you buy it from the Apple AppStore.
Some professionals, for example the famously grumpy Scott Bourne, have decided that Aperture is no longer their favorite package, even though they have used it for years, and have written books, given lectures, etc. See http://photofocus.com/2012/07/20/aperture-v-lightroom-built-in-noise-reduction/
Apple Aperture does support Retina display. A version of Lightroom supporting Retina is on its way. So in the end you should choose whichever application you like the best.
I would suggest that you download demos of both applications and try them out for yourselves.