Q: Could I start seeds in trays on my enclosed porch if I bring them in at night? Also, do I have to buy potting mix, or can I start them in compost?
A: You can try starting seeds on the porch, but even a sunny porch probably does not provide enough light to get seedlings off to a good start. And when the sun is fierce, it can dry out your soil and make the plants too hot. Your seedlings won't like it if they get too cold, either. They should maintain at least 60 degrees.
We recommend using artificial light in a more controlled environment.
You don't have to use a grow light. A simple shop light from the hardware store, hung from chains so you can keep it a couple inches above the top of the plants, should work well. Choose cool white bulbs, or one cool and one warm bulb.
We don't recommend starting seeds in straight compost, for a couple of reasons: If you're buying compost, it can be more expensive than a mix of compost and peat, and since there's no advantage (and there are drawbacks) to using straight compost, you're better off saving your money. Compost also doesn't hold moisture as well as potting soil or seed-starting mix.
You can make your own potting mix that includes compost, but if you're going to start seeds in it, you need to be sure the compost is mature or you risk damaging your seedlings.
We recommend using a soilless seed-starting mix, usually a mixture of peat and vermiculite. Make sure it's damp but not soggy before you plant your seeds. I like to use warm water to dampen mine, and I use an electric heat mat for bottom heat to encourage germination.
Your porch might be a good place to harden off your seedlings when you're getting close to planting them in your garden. Seedlings need to toughen up before they can be planted out, so it's important to gradually expose them to sun and wind, a little more time each day, for a week or so before planting.
There's lots more information about starting seeds here: extension.umn.edu/planting-and-growing-guides/starting-seeds-indoors#bottom-heat-1179612.