Hola and welcome to the Forum
Your questions are ones that everyone asks! The answer is- it depends!
In term sof the the choice of foortwear it depends on the time of the year and what the weather might be like. You would wear goretex lined boots or mid ankle shoes in spring and winter when it is likely to rain and lighter walking shoes in summer. Nowadays most people avoid leather boots because of the drying time - but there are some who still swear by them. This is one subject on which all walkers have an option! If you type "boots" in the seach box a number of threads will come up with lots of information on this subject. But don't hesistate to ask here. A starting point might be going along to a good hiking supply shop and explaining to them that you want to walk the Camino and what time of year you may go - they will assist. My advice would be Don't Buy First Time - gather information. Go along in the afternoon mid week - you feet will have swollen with the normal walking of the day and you are more likely to get a sales assistant with expereince rather than a weekend temp. Try on some boots and shoes (with your own socks or the socks they provide) and walk up and down the little hill they provide. Take your time and go back until you have settled on that you want.
In terms of training in advance - I think if someone if very unfit and overweight for example it is advisable to get a little more into shape or there will inevitably be muscle and joint problems. It is always a good idea to do some training walks to get your feet used to walking with shoes or boots. A few days hiking with a full rucksack is also good.
If you are walking the
Camino Frances however keep in mind that there are albergues at close intervals and it is possible to start more slowly and build experience and stamina over the first week or so.
You certainly don't have to be an athlete!
Best wishes
John