Find wood

You put an item into the scanner and press scan. If nothing happens, the item cannot be scanned (or it is not coded yet since it is a beta).

If the item can be scanned, it will be destroyed and be made available for 3D printing which does cost pixels to perform. Pixels can be obtained by finding them in containers, by killing enemies, or by refining ores when you can build the Refinery.

Items that are known to work so far are various kind of seeds, like Grain or Banana Seeds.

Some trees (which may not be present on every planet) yield plant fibre when cut down. Just use your matter manipulator.

Vines work as plant fibre. You can find those growing on cave ceilings. Cut them with the matter manipulator, too.

You can grow some yourself, which is renewable, but takes some time and requires some preparation. You'll need a few seeds (you start with those) and basic stone tools.