Soil moisture from sentinel-1 (GRD Data)

FAQ: What are the minimal requirements for SNAP?

It depends a bit on the spatial distribution of your sensors - how large is your agricultural area?
The easiest way is to plot measurements from the field against the calibrated backscatter values in a scatter plot and derive a regression equation. Please have a look at these topics:

Pre-Processing is comparably easy, the most important steps are radiometric calibration (and conversion to dB) and geocoding: Radiometric & Geometric Correction Workflow - #35 by ABraun