Starting your own plants from seed is often treated as a mysterious and bedazzling craft that is up there in the same stratosphere as wizard work. It's not. It does require a little learning and more patience and attention, but other than that it's pretty simple and straight forward.
A seed wants to grow – it's not like you are trying to get your teenager to clean up their room – a seed really wants to grow and fulfill its mission in life. All you need to do is to provide the right conditions, and keep providing the right conditions, until the plant is definitely on its way. And, of course, at that point, you are keeping it alive in the garden. You see, it's not that much different.
Choosing your own seed allows you not only to choose varieties your local nursery isn't carrying, but allows you to choose multiple varieties that extend the length of your harvest or the breadthy of your harvest. It allows you to be organic from the very first drop of water the seed imbibes all the way to placing it before your family and friends at the dinner table.
All you need is some good seed catalogs and a little self-restraint (don't buy hundreds of dollars worth of seed at once – a little here, a little there would be much more realistic!); a potting medium (garden soil is NOT a potting medium for any plant and especially seedlings!); a place to make a mess and a bright area that gets enough sun (or light of some kind) to sprout seeds.
Long before you begin, go on some websites of seed purveyors and obtain a current catalog - get as many catalogs as you feel comfortable having and peruse them to your heart's content. What do you want to grow? What qualities do you admire in a tomato or a squash? Choose only a few the first time out. Tomatoes, cucumbers, squash, beans, peas and other common vegetables are the best for learning. Order single packets! Here's where I like Pinetree Garden Seeds in Maine; they have a good selection of seed, but each package is packed with fewer seeds and sells for less – for most home gardeners, this is a godsend! I want to grow several varieties of tomatoes and don't need but a few plants of each, Pinetree makes that more possible and don't require an expenditure similar to a down payment on a home.
In reading the catalogs, you are looking for traits that suit your needs – I like to grow paste tomatoes, early tomatoes, and a couple of cherry tomatoes. I look all through my catalogs and compare maturity dates and size and descriptions of productivity to find the best finalists from which I will whittle down to my nine 'gotta haves' for this year.
The easiest way to do this is to place your seedlings in a bright spot that won't be too cold and is protected from critters of all kinds. This can be a simple table under a shade tree near your back patio, on the back patio, or, if all else fails, in a spare room in the house with fluorescent lights and a timer. I have done them all - including starting some plants in a professional greenhouse. It's still the same formula with slightly different tweaks.
For containers, I typically recycle six-packs, four inchers (sometimes called quarts) and one gallon nursery pots. I also have some small terra cotta pots I use, but the plastic ones are (sadly) a lot easier. I fill these up with potting soil – if I can't find a brand as smooth as I want, then I sift out the big pieces and make a finer mix that way. Fill the chosen pot almost all the way full with the potting mixture and press down firmly. You will want to end up with a container in which the potting soil is below the outside edge of the pot – when you water, you want standing water on the top that remains there until it sinks in; a fully topped off pot would allow the water to run off without wetting the soil.
This will lead you to the next step nicely and smoothly – there is little chance you will ever learn to save seed if you don't grow your own plants from seed. I think this is essential to really becoming a gardener. In fact, most gardeners, who think they are good, will sheepishly confess when they don't feel competent growing from seed; like they know they haven't quite lived up to their own standard of being a 'good gardener'. It takes some learning to grow from seed, but it's not impossible or difficult – any more so than any other part of gardening. And once you learn to do it, you will be proud of your skill level and the plethora of choice you now have.
In the near past, some plants were grown in diseased greenhouses and the entire tomato crop in the east was affected. Don't import insects and disease into your garden - start from seed and have a good harvest!