It really depends on how much time you have available to walk the Camino, and which Camino you plan to walk - there are several routes to Santiago de Compostela.
The modern traditional starting place on the popular Camino Frances is either St Jean Pied de Port on the French side of the Spanish/French border, or Roncesvalles which is 25kms away on the Spanish side of the border. This gives a distance of about 500kms. Most people allow six weeks to walk this distance, but faster, stronger walkers can do it in one month or so.
If you have more time available, and are feeling energetic, you could walk from one of the four medieval French starting points - Paris, Vezelay, Arles, or Le Puy en Velay.
If you have less than six weeks available to you, then consider starting closer to Compostela, perhaps from one of the larger cities such as Pamplona, Burgos, Leon, or Astorga. It's really up to you.
I really suggest you have a look at a couple of Camino websites, there are several, and a couple of ones I like are the Confraternity of St James, which also publishes a Guide to the Camino Frances, containing distances between towns and villages, number of albergues in each town with number of beds, list of private albergues and hotels, if the village contains a shop, and other useful information; and CaminodeSantiago with a website containing plenty of information for pilgrims, a forum where you can ask questions, and also a Facebook page.
So, just work out how much time you want to spend, the amount you're likely to walk each day, and then work out a starting point from Santiago de Compostela. Just remember that, if you want to get a Compostela certificate from the Pilgrim's Office in Santiago, you will need to walk the final 100 kms.