Hi Darren, that is an awfully big question and you might like to try and refine it a bit. Type "Packing List" into the search facility and you'll find some pretty good suggestions, and some lively disagreements too. Use the search facility also to access opinion on any Camino related topic and quite a few other topics as well. And feel free to ask in the Forum. Lots of members will try to answer.
Here I my suggestions, in no particular order, for things you might find useful to do before you take those first steps:-
Take some practice hikes in the boots / shoes / sandals &c that you intend to wear on the Camino.
Pack the kit you intend to carry into the rucksack you intend to use and take some more practice hikes.
Read up on the history of the Camino, of the great cities and other cultural sites you will pass, and of Spain if that sort of thing interests you.
Learn a little Spanish: the common courtesies, some menu stuff and how to ask for and understand directions will help.
Get your credencial, you can order one here from Ivar.
Choose your guide book / App and familiarise yourself with its foibles.
Decide on your starting point.
Start.
And Buen Camino