If you're starting your own seeds in trays, you should know that you're not supposed to use regular garden soil. You need to use a "medium" that is sterile, meaning that it doesn't have fungus and bacteria that will be hard on your tender little seedlings, especially in the humid conditions that are ideal for seed starting.
You can buy seed starting mix at your local garden supply store or, as I've learned, you can make your own that is just as good if not better. It will take an initial investment but if you're going to get into starting plants I think it's worth it.
The basic ingredients are vermiculite, perlite, and peat moss. Buy the big bags for around $25 each at Northwest Seed and Pet, and you'll be stocked up for several growing seasons. The basic mix is 3 parts peat moss, 1 part perlite and 1 part vermiculite. I make my batches by using a plastic pitcher, and I toss into a dedicated garbage can 9 pitchers full of peat moss, 3 of perlite and 3 of vermiculite. Note: use a particle mask while doing this. I usually wet it down a little before mixing it to keep the dust down.
Make sure to mix it up well, wet it all down so it's damp, but not soggy, and it's ready to load into your planting trays. It's much easier filling plastic trays with soil that is already the proper dampness.
I learned this mix from Bruce at GEM Garden and Greenhouse. He sells this medium with some other goodies added in for a great price. If you're only starting a couple of trays you might want to go that route.
Seedlings supply their own fertilizer for the first week or so as they feed off the remnants of the seed. After this they will need some mild fertilizer input. I use Osmocote slow release pellets that I scatter around the top of the soil, (not touching the stem), so every time I water they seedlings they get a small dose of fertilizer. It's not organic but it sure makes it easier for me. I have this phobia of chemicals so I like not having to deal with applying fertilizers. I'm open to ideas on this from someone who has organic fertilizer's figured out.