While it may not always work out cheaper financially to grow your own, you will know for certain how the produce has been grown (e.g. spray free or fully organic).
Save and plant your own seeds where possible (cheaper than buying seedlings).
Composting is a responsible way of recycling organic waste.
Swap excess produce with neighbours – even better, plan ahead with your neighbours so you don’t all grow the same crop.
Gardening books can often be purchased for very little from opportunity shops. The internet is also a good source of growing and harvesting information.