I just don’t know where to start ... any info is much appreciated
I would start with prepartion of both your body, and mind. It may be straight forward to walk between 20 to 40 km severl times a month. To my mind it is another thing to do that day after day for a around 30 days.
I'm a slow learner. It took me nearly a year to regularly achieve 30 km each outing. It took a lot longer to achieve 20 km non-stop before a breakfast stop, as a way of training my mind.
I am lucky that the topography where I live gave me two routes where I could attain between 500 and 700 metres of elevation gain, the latter before breakfast.
And part of preparation is finding what pack, clothing and other stuff works for you on your training trips.
While preparing mind, body and equipment you think about your route that will (eventually) end at Santiago de Compostella.
Tincatinker has blown you away with the large number of routes.
A major factor for tyros is knowing the infrastructure, especially accomdation and food will be avaialble at regular intervals. So Kanga, rightly, suggests the Frances starting at Saint-Jean.
Depending on your closeness to western Europe and time avaialble I make three suggestions.
Firstly, a short Frances starting at Sarria. This can be seen as an extension of your preparation.
Secondly, to start at Le Puy-en-Velay.
Thirdly, longer routes could be completed as a continuous journey to Santiago Or completed in segments of, say, two or three weeks at a time.
Kia kaha (take care, be strong, get moving)