Things to consider:

Ventilation – Plants don’t grow well above 30 degrees C.  Ensure ventalation on the roof (hot air rises)  As well as neas the bottom to draw in cooler air

Temperature – But a max/min thermomenter so you can monitor overnight temperatures

Light – Wash greenhouse to remove accumulated grime that cuts down on the amount of light reaching plants.  You may also want to provide shade in summer

Watering – Set up an automatic system or water by hand.  Check regularly.  Tomatoes may require more than once a day

Heating – a range of heaters and heat pads are available commercially or consider insulating in winter with plastic film, polystrene sheets or frost cloth.  To grow tomatoes try to keep minimum night temperature of 12 degrees C

Pests and diseases – mites, thrips and whitefly can be a nuisance.  Be vigilant and take action at first ign of a problem.

Site – is a building consent required?  Place on a level, well drained spot facing north.  Close to a water supply or add plumbing to the greenhouse.

Glass or Plastic?  As a rule glass lets in more light than plastic and retains heat better.  Plastic is cheaper.  Plastic deteriorates so will need to be replaced.