Technically speaking, you're playing Mother Nature. However, if you've never grown in a greenhouse before it's always best to take that beginners approach by growing springtime plants in spring, summertime plants in summer and so on.
Once you get the knack of how those kind of plants act and react while growing in the greenhouse then you can slowly incorporate planting unusual things that are typically not growing in their growing season.
It is also not a bad idea to experiment with growing seeds as well as sets. This way you'll get a better idea on whether or not seeds do best for you or sets.
And don't forget, growing plants other than edibles in a greenhouse is another form of gardening that can't be overlooked.