From Atlanta, you should be able to get a direct flight to Paris (CDG). Ideally, if you can get their early morning, you would have time to make it to SJPP the same day. However, as it is a relatively sleepless flight across the Atlantic, you might also think about spending the day and night in Paris (particularly if you’ve never been there before. Whether it’s the same day or the next day, you’ll want to make your way to the Gare Montparnasse to catch the high speed train (referred to as the TGV) to Bayonne (there are several every day). From there, the one-hour regional train will bring you rest of the way to SJPP. Because you end up in SdC, it would be easier to schedule your return flight out of Madrid. Getting from SdC to Madrid is quite easy by either train or plane.
Although April 23 is still quite aways away, a couple travel ideas to keep in mind:
1) Booking your flight as early as possible usually gives you the best rate.
2) Although it’s much too far out now, again to get the best prices, book your European train/plane travel well in advance.
3) Do think about reserving beds the first few days of your Camino at least up to Pamplona: SJPP, Roncesvalles, and Zubiri/Larrazoaña.
4) It’s not too early to think about your gear and to start pulling things together. My recommendation is that you don’t want to check your pack with the airlines. Keep your pack size and weight down so it can travel in the compartment with you (I happen to travel with a 36L pack thst has always been allowed in the cabin with me). If you were to leave Paris or elsewhere immediately, you don’t want to chance that your pack doesn’t arrive.
5) Finally, enjoy the Camino planning process. As great as the walk is, getting ready for it with all the anticipation that accompanies it will make it even better
Buen Camino!