just get a reservation for a TGV to Irún (twice daily) in Paris at the station for 6€. If these two trains are sold out, you can go by TGV as far as Bordeaux or Dax and then continue by regional trains (TER) without reservation. Doing the whole trip from Paris with trains with no reservation is almost impossible within one day...I would not recommend it..
Another alternative would be the night train to Irún (6€ a seat, 20€ a couchette).
From Irún, Cercanias suburban trains run about half hourly to San Sebastian. If you dont use a TGV or the night train south of Bordeaux/Dax (that means if you are travelling with a regional train) you will not get as far as Irún - these trains stop on the French side of the border in Hendaye. In that case you have to use Euskotren light rail to cross the border, a ticket is about 1€. Euskotren calls in Irún where you can change to San Sebsatian. Or, if you want to invest a little more money, you can go with Euskotren all the way to San Sebastian as well.