disc brakes start squealing after just a bit of use
A couple things to look at:

Perhaps try Brake Clean to clean the rotor and spray on a rag then wipe the pad surface. I successfully use it on rim brakes when they occasionally squeal and it is not due to the toe angle of the pads. However be careful with it as it is stronger than rubbing alcohol so do not get it all over the place.
It could be possible the rotor is out of true. In which case, you should go the shop you got the bike at to get that addressed.
You could possibly have a bubble in the brake line which is causing a lack of pressure to close the caliper and that in turn is causing vibrations as you slow along with the loss of braking power. Again the shop that you got the bike at should be able to address that for you pretty quickly.

Hope that helps.