There are so many factors that go into where you start your Camino that it is hard to give definitive advice. It depends on how long you have to travel, what is important to you, your level of fitness, etc. So I'm going to make some assumptions, state them explicitly, and if I have any of them wrong, you know where to adjust my advice (although, of course, you can always ignore it entirely).
I'm assuming that you want to walk as much of the "traditional"
Camino Frances, famed in books and movies, as possible and have plenty of time to do so. That's why you are thinking of starting in SJPdP. The "plenty of time" is a crucial part of my assumptions. My general recommendation is always give yourself more time then you think you will possibly need. That reduces the possibility that you find yourself racing towards Santiago to catch a plane, never a situation you want to be in. If you don't have plenty of time, you might want to start a bit closer to Santiago. If you find you have extra time at the end, you can walk to Finisterre/Muxia, or there is plenty to see in Santiago and Iberia. Since this is your first Camino, you don't really know how far you can walk day after day before the wear and tear on your body becomes excessive. With my teenage son, we found that 20-25 km a day worked fine but 25-30+ km a day, day after day, inevitably produced terrible blisters. Who knows what it will be for you?
But I'm assuming you have the time and want to walk the "whole"
Camino Frances (and don't get me started how there really is no "whole"
Camino Frances!) . My advice for a first timer would be:
- Take two days to get from SJPdP to Roncesvalles. If the Napoleon route is open and you are taking it, stop at Orisson. If you are taking the Valcarlos route, stop at Valcarlos.
- Book the first few nights in advance: SJPdP, Orisson/Valcarlos, Roncesvalles, perhaps Zubiri if you plan on staying there after Roncesvalles. After that don't worry about booking ahead. Maybe towards the end in Galicia you will want to book a day or two in advance again. By then you will know how far you tend to walk in a day.
Personally, when I have walked, I walked from Roncesvalles and last time (July 2016) I booked Roncesvalles and Zubiri. Next time, I am likely to start at Ostabat (a day before SJPdP where the routes from Paris, Vexelay and Le Puy join). In that case, I would reserve at Ostabat, SJPdP, Orisson, Roncesvalles and Zubiri but not beyond. If I were walking from Le Puy or Vezelay or Paris, I wouldn't stop at Orisson. By that point in my walk, I think I would be conditioned so it would be a lot less challenging to walk from SJPdP to Roncesvalles than it will be on the first or second days.