It can be a pip from an apple, an acorn from an oak or a winged nut from a maple - you can even go crazy and buy a whole packet of tree seeds from your local garden centre if you want.
All you have to do is make sure they are the seeds of trees that found naturally - or "native" - to the place in which you live.
But where do you plant it?
On the side of the side of the street, in your garden, in the park, by a lay-by, on the side of a field or next to a wood... anywhere where there's space and light. Then all you have to do is
- bend down,
- push your finger into the soil about to about 2cm (1 inch) depth - or use a pen - and pop that seed in.
- Then stand on the hole to close it over and tuck it in.