Greenhouses are typically not heated and being glass, they are not insulated. That means they warm up in the daytime and provide some protection against wind chill, but at night will not be a lot warmer than the outside. You can improve this a bit, by putting something like a black painted 50 gallon drum full of water in it. The water will absorb heat in the daytime and give it back at night, but still you would be doing well to keep a small green house a few degrees warmer than outside.
You can heat the greenhouse, but given the lack of insulation, it's a very energy consumptive/ expensive proposition.
So you would probably need to start things indoors under lights and move them to the greenhouse later.
If you are a beginning gardener with not a lot of experience starting seeds and growing things, I would really suggest you get a little more experience before investing a lot of money in a greenhouse. I don't have one and have gotten by without for many years of gardening. It seems to me that the greenhouse only makes a difference for a few weeks in spring and again in fall. Don't need it in late spring, summer, early fall and can't use it in winter.