Andrew, exactly how you remove field tiles will vary a lot, depending on the actual make of the tile, and the installation. Some interlock; some don't. Some are brittle; others are quite sturdy.
As an overall though, assuming these are clay or cement barrel tile, they will have a lip at the top edge which "hooks" them over 1x2 slats nailed to your roof decking, and may have one or two nails near the top, too. You have to work pretty carefully to get the first few tiles out, then you've got some room to work them.