Paris Gare de Lyon is the station for trains to the southeast.
Use DB Navigator app or any other planner. The logical route is via Bellegarde - Annemasse.
Cambronne is a metro station, not included in the Eurail pass (as it is urban transport).
You’ll need to get to Paris Gare de Lyon.
Most convenient option would be:
Paris gare de Lyon, at 06:50 TGV 6467 to
St-Gervais-lesBains-le-Fayet, arrival 11:34, departure 12:28 to
Chamonix-Mont-Blanc (With a TER to Vallorcine) arrival 13:13
Or a bit later:
Paris Gare de Lyon dep 10:31 TGV 6473
St-Gervais-les-Bains-le-Fayet, arrival 15:36, departure 16:35 (by bus)
Chamonix-Mont-Blanc: arrival 17:00
You need seat reservation for the TGV.
Reserve your TGV trains at www.raileurope.com , you can also check the train times as well there.
If this is the only travel you will make, you might find that normal tickets are cheaper than a Eurail pass.