You want to start a garden, but you've no money to spare and no idea where to start. One way is to join a gardening club, details can be obtained through council, library or most have Facebook pages. It's a good way to meet generous, helpful people who are likely to give advice and share plants with you. Next are plant swaps, but when you have no garden, there's nothing to swap, so find a local garden and offer to help in exchange for plant material. This can be a great hands on way to learn about gardening and propagating plants. most gardeners love to propagate, so they've always got some spare plants available.
Starting this way is in itself part of the sustainable garden ideal, pots are reused, new plants are made from old, and the time you'd spend travelling to source plants is reduced. Add to that the social benefits, and you're on a winner, and well on yoyr way to becoming a gardener.