What you can grow in your greenhouse is only limited by warmth in your greenhouse. First, select an energy source. Electric heaters are generally less expensive to purchase and work well when you have electricity to your site. (Extension cords are not a good idea in a greenhouse setting and could prove unsafe.) Another option is gas heaters, either natural gas or propane, as they are inexpensive to operate and require no electricity.
If your goal is over-wintering plants, you will only need to keep your greenhouse above freezing. If you plan to get an early start with seedlings, you will need a more consistent and constant heat supply.
Heaters are rated by the amount of BTU (British Thermal Units) they produce. To determine the correct size of heater, you'll need to determine the size of your greenhouse, the expected minimum outside temperature for your area, and the minimum temperature you want to maintain in the greenhouse.