1. Install Google Earth from http://earth.google.com . Open Google Earth and go to Tools\Options
2. Select Show Lat/Long \ Decimal Degrees and click Apply
You only do steps 1-2 above once on your PC!
3. Have an Excel file with your location addresses ready
4. Select and copy "adress" cells. That can be exact street address or zipcode, or county, state. Paste it in Google Earth search box and click on magnifying glass. It will zoom you to the respect location. Now zoom to specific farm/field. When you place mouse pointer on the place, XY will be displayed.
Alternatively you can set a push-pin (Placemark) on the trial and copy Lat/Long from pop-up box below.
Type Lat/Long into Excel table for the location. Repeat for all locations in the file.