I like to use a plastic cover that fits over the flat that holds the pots or cell packs. They sell seeds and plants too. Once plants germinate and you see green leaves, prop the cover up, and then remove it completely in a day or two. The potting mix has enough nutrients to get the plants growing, but you'll probably want to supplement with a liquid fertilizer as they continue to grow. I prefer to use a liquid fertilizer at half the recommended strength, either every time or every other time the seedlings are watered.
Sufficient light is perhaps even more important.
If seedlings are bending toward a window, or growing thin and spindly they don't have enough. Keep seed trays within a couple inches of fluorescent light tubes on a grow light stand. If a window supplies the sunlight, reverse the tray to keep things evened out. When your seedlings develop two or three pairs of real leaves, not the rounded baby leaves that first emerge from the seed, it is time to transplant. The idea is to space them out and give them more room to grow without crowding.