My house is new, but in the bathroom between the ceiling and the wall above the bathtub/shower a small crack is forming. Is there a good and easy way to fix it before it gets worse?
Wait till the house is at least a year old to fix minor cracks.
The most likely cause is the framing drying out (and shrinking) in the now conditioned space of the house.
You can retape. This would involve removing the old tape or taping over the top. I would recommend a high strength tape as discussed in the link below. I appears you have some sort of structural stress, assuming the joint is taped.
Here is a link that might be useful: Retaping Drywall With High Strength Tape
If you fix it before about 12 months be ready to fix it again.