If you're only going to learn to accompany one Sevillanas, A La Puerta de Toledo is a good place to start. It's by far the most commonly sung Sevillanas, and the chord changes are relatively simple. Once you've understood how this one works, any other Sevillanas you want to learn will be a matter of learning the chord changes. This is also a great introduction to how Sevillanas work from the point of view of the singer.