You can use this useful Journey Planner to work out your best route and book tickets if you want:
Stations in Somerset include Bridgewater, Frome and Taunton.
Taunton is the best starting point if possible as you can get a direct service to St Erth, then change to the St Ives train on the branch line. Details of the short journey from St Erth to St Ives are on my website:
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