Tiny dry tips on a Calathea plant are perfectly normal and also seen in nature. It is also quite common for the odd lower leaf to turn brown and shrivel up, just remove these lower leaves. Calatheas enjoy light but not direct sunlight, as this can cause their leaves to scorch. Also, keep this plant away from draughts as they also don’t like to be too cold and this might cause some of the leaves to drop.  Generally, Calatheas like to be kept quite moist - don't let them dry out for too long as their leaves curl up.