No matter the size or type of greenhouse, they all share one common problem: pollination. Outdoor gardens have bees to pollinate them. When you have a greenhouse, you have to do it yourself. Pollinating greenhouse plants by hand takes time, but no special skills. In some cases, you may not even need to carry any extra tools with you into your greenhouse.
Observe your plants as they flower and start to produce pollen. If you do not already know, research the plant to see if the species in question has separate male and female plants.
Swab a little of the pollen from the stamen carefully with a clean paintbrush.
Apply the pollen to the inside of the flower you want to pollinate. This is called the carpel, or pistil. Some plants are self-fertile, meaning they reproduce without another plant. On these plants, move the pollen from the stamen of a single flower into the pistil of that same flower and it will reproduce. Self-sterile plants require you to use pollen from one plant to stimulate reproduction in a separate plant.
Repeat the process for all of your plants as they come into bloom. Use a separate paintbrush for each species, and wash them well between uses.