Don't start too soon or you could end up with leggy plants ready for the garden long before it's safe to transplant them. I've found that when sowing instructions advise starting seeds four to six weeks before the last frost date, it's best to go with the four weeks. Another pitfall is sowing more than you need. The first year, I used up an entire packet of foxglove seeds and ended up with seven dozen plants hogging precious space under my three shop lights. By all means, plant a little more than you need to ensure that enough seeds germinate, but then thin them out to a manageable number.
Many leftover seeds are viable for several years if stored in a dry, cool, dark place.
For sowing, use a commercial seed-starting medium, a soilless combination of peat, perlite and vermiculite. Moisten it an hour or so before sowing. As general rule, plant a seed about as far below the soil as it is large and barely cover very tiny seeds. If seeds need light to germinate, just press them into the growing mix. Give all seeds a light sprinkle of water and cover with a plastic dome lid or seal the container inside a plastic bag.