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It really depends on the plant, but the general rule of thumb is 4-6 weeks before your last frost date. Don't worry if that time has passed. In Ohio, you can wait as long as the end of May to plant in the ground and the plants will catch up quickly, so you really have until the end of April to start seeds without hurting the plants in the garden.

These articles should help: https://www.gardeningknowhow.com/garden-how-to/seeds/when-to-start-seeds.htm, https://www.gardeningknowhow.com/garden-how-to/seeds/cheap-seed-starting-how-to-germinate-seeds-at-home.htm, https://www.gardeningknowhow.com/edible/vegetables/vgen/growing-vegetables-with-seeds.htm

Really any type of heating pad will work. Just be sure to put it on the lowest setting with a towel on in. These articles should help with starting your seeds: https://www.gardeningknowhow.com/garden-how-to/seeds/cheap-seed-starting-how-to-germinate-seeds-at-home.htm, https://www.gardeningknowhow.com/garden-how-to/seeds/soaking-seeds.htm, https://www.gardeningknowhow.com/garden-how-to/seeds/when-to-start-seeds.htm

This is simply a best used by date and does not necessarily mean you have to use them all, though it's recommended. As long as you store the seeds in an airtight container until ready to use again, then they should be ok. Do not freeze the seeds though, as this will kill them.

Typically if they are not marked, they are hybrid. Organic and non-hybrid are different, so a seed can be organic but hybrid too. You may want to buy seeds from companies that specialize in non-hybrid seeds, like rareseeds.com or seedsavers.org.