Decide what type of greenhouse you want based on the space that you have. You don't want to fall in love with a particular design and then learn that you don't actually have the room for it in your home. \
Measure the space where you're thinking about placing the greenhouse so you can choose a greenhouse that's going to fit in that space well. \
You can always start with a small greenhouse and upgrade to a larger one later if you decide you like having one in your home and want to expand. \