Many questions that we get in the forum are related to “I am flying in from X and need to get to my camino starting point Y” and then the conversation goes on how to best travel within Spain.
Early on we created a section in the forum, just for those kinds of questions and we have over 1600 posts with suggestions and questions on this topic.
Many of these questions includes mentions of either a bus company or the train in Spain. But it is also worth mentioning that you can get some good prices and deals on domestic flights in Spain. Many times these flights are cheaper than bus/train, and takes a fraction of time.
Have a look at the forum for how you might travel to your starting point.