For most people in the northern hemisphere it is time to start your seeds inside in order to have plants large enough to transplant into the garden after the danger of frost has passed. People with short gardening seasons may need to start most of your plants indoors, while those of us in sub-tropical areas can plant many of our seeds directly into the garden. However, even in Florida, we start our tomatoes, eggplant, and peppers in seed trays to give them a head start.
All you need is a sunny windowsill, trays, pots, sterile seed starting mix and seeds. I'm not getting into how to treat each individual type of seed; just read a good gardening book for tips.
Observe your chosen seed starting area; it should get at least eight hours of sunlight each day. If not, you may need to move your trays around the house to follow the sun or use grow lights. Those of you with greenhouses don't have to worry about the amount of sun, but should make sure that you monitor the temperature to avoid overheating.
Fill your pots with sterile potting mix. I use new mix each year, but some people like to bake their old mix or a self made mix in the oven. I believe the temperature should be 350 degrees F for 1 hour. If anyone does this please correct me if I am wrong. I don't sterilize mix in my oven because the smell is intense.
Plant your seeds according to the seed packages directions in the pre-dampened potting mix. Large thick seeds may need to be pricked with a knife and soaked overnight in water before planting.
Place the pots in your trays and cover them with a clear plastic cover. The plastic should not touch the top of the pots at all. This is your mini greenhouse, but watch carefully; as soon as the plants germinate you want to remove the covers.
Keep your soil mix damp, but not wet. If the mix is kept too wet your plants may succumb to a fungal condition called "damping off". If the stem of the plant narrows and rots at the soil level you have damping off disease. Throw it away and don't reuse the planting mix until it is re-sterilized.
If your plants are growing, but are tall and spindly they are not getting enough sun. I will take my trays outside if the day is warm so they get a nice steady source of sunlight.
Your plants do not want to move from their nice warm spots to a chilly garden and they will protest unless you take the time to harden them off. Start out by putting them out in the yard in their trays for a short period of time and gradually increase the time for about a week.
Once the danger of frost has passed and your seed starts have a good root system you can transplant them to the garden. Your hard work will pay off; your plants will have plenty of time to grow and produce fruit before winter freezes begin.