Andorra has several international airports at less than 200 km away, look on the location map. Search, on the corresponding section the bus lines that connect them to the Principality and, in case you rent a car, the best route for coming.
From Andorra you can get to the Spanish (RENFE) and French (SNCF) railway stations.
Look for the nearest station on www.mobilitat.ad, on the map and the bus lines that connect to Andorra. You also have the possibility of renting a car.
Several cities in Spain, France and Portugal have a daily bus service with the Principality of Andorra.
Check companies, times and prices: www.mobilitat.ad
Remember that Andorra has an efficient public transport network and several car rental agencies.