You have several options to go to Cadaqués. For me the best is to take a bus from Barcelona from the North Bus Station (they also stop at Urquinaona square). This bus takes you right there, to the parking lot which is few minutes away from the centre of Cadaqués.
Also you can take a train to Figueres and from there there are buses to Cadaques.
Transportation in Cadaques is quite simple if you are a tourist...you walk everywhere. Several of the locals ride around on scooters, and they ride FAST! I met a few people who took me around on these, and they drive like maniacs up and down the steep narrow streets! However, it is super fun!
There are no cabs, and if you need a ride to or from the bus stop, you will need to arrange it with your hotel in advance.