Andorra is nestled in the Pyrenees mountain range, and the only way to get in and out of the country is via road: one road leads to France, the other road leads to Spain
We drove from Nice to Andorra in 1 day - left Nice in mid-morning, and got to our hotel in Andorra in the evening. So it's quite a long drive.
In Andorra itself, I don't think there's any public transportation. Either way, if you're going to ski, it's much better to have a car since the the skiiable terrain is HUGE and there having a car makes it so much easier to go to different areas (and you can stay in cheaper hotels away from the village)
Parking got pretty tight, so get to the hills early
Gas was a bit cheaper in Andorra than France. Fill up before you leave
We left Andorra and drove to Barcelona in 1 day. The drive was pretty scenic. We left first thing in the morning and got to Barcelona by mid afternoon.