First step would be the shop as mentioned.

If you want to look it over just to get a feel for it then you could unmount the rear wheel and check the freehub for drag and for proper freewheel pawl operation. It's possible that it's a little draggy for a few different reasons. If you spin the cogset and freehub smartly it should spin for quite a few turns. If it doesn't spin at least a half dozen then something might be rubbing or it has too thick a grease like lube in it. But that's something the shop should be checking before you start tinkering on a new bike. It's OK for you to pull the wheel off and give it a once over. But stop before you reach for any tools for the sake of the warranty.