Deciding on what you will grow is one of the exciting parts of gardening. Pouring through catalogues and seed packets at the plant nursery always make me over-estimate how much ground we have and under-estimate our time. You need to be prudent, especially in the first few years of gardening. My advice is to write down a list of what vegetables you're buying and make up your garden list from that. If you're new to gardening, plant the easy things first, then work your way into the more difficult vegetables. These vegetables are easy to grow: beans, peas, tomatoes, squash, pumpkin, cucumber, zucchini (courgettes), corn, lettuce, sugarloaf cabbages, chard, kale, spinach, most herbs, carrots and radishes. If you're in a hot area, luffas, peppers and sweet potato will grow well; if you're in a cooler zone, cauliflowers, onions, broccoli and all types of cabbages will do well. If you're still undecided about what to plant, work out what you're buying, then choose the most expensive vegetables, or the ones you eat the most of.
Seeds may be planted out into the garden when the soil is warm enough (you'll find that info on the seed packet), or in trays that can be transplanted when the plant is big enough. Beans and peas and root vegetables like carrots and radishes are best planted directly into the ground because they don't transplant well. You can plant carrots and radishes seeds mixed together. The radish tops will appear within a week, showing you where the carrots and radishes are. When the radishes are ready to harvest, you pull them out and that gives the carrots more room to grow.
Most fruiting vegetables - tomatoes, zucchinis, squash, peppers, capsicum, pumpkins etc and leaf vegetables - lettuce, cabbage, kale, chard etc are easily transplanted so those seeds can be planted in trays or pots and raised in a warm place. As soon as the soil is warm enough you can transplant an already growing plant, saving the three or four weeks it would take the seed to reach that stage if you planted it straight in the ground.
A seed is a self contained unit of nutrients that only needs warmth, moisture and soil to germinate and grow. When you plant your seeds they do not need light or fertiliser to grow. Placing them into soil and watering them gently is enough to make that seed burst into life. When the seed has germinated and has grown the first couple of leaves, it then needs adequate light, moisture and light fertilising to keep it going. Compost tea, comfrey tea or organic liquid fertiliser are all ideal for this task. You make those teas by adding a quarter bucket of either compost or comfrey to a bucket of water. Stir and allow to sit for a day or two, then strain the compost or comfrey out and keep the liquid (the strainings go in the compost). This is then diluted one part tea to six parts water and used on the seedlings after they put on their first few leaves.
You plant the seeds according to their size - each seed is planted at double it's size. For instance - if your seed is 1 mm, plant it 2mm deep, or one eight of an inch, plant it one quarter inch deep - so the bigger the seed the deeper it is sown. The instructions for planting seeds are on their packets. Make sure you plant into clean containers using seed raising mix or your own light blend of garden soil, sand and old compost - is must be well draining and it must not have any lumps that might stop a seed from emerging.
When your plants are ready to plant out, plant them at the same level they were growing in the pot or tray. The only exception to this rule is when you plant tomatoes, you remove the bottom set of leaves and plant it deeper than it was growing in the pot. Tomatoes can send out roots from all along the stem, so planting them deeper allows them to grow more roots. More roots = more tomatoes.
Plant out into well prepared soil that contains a lot of organic matter. The exception to that rule are carrots - they like a bed that has little compost and no lumps. When you transplant your seedlings, water them in with a weak solution of seaweed concentrate that has been diluted according to the instructions on the pack. This eases transplant shock and will help your plants grow well from the first day. After a week, when you know the plants have settled in well, start your fertilising regime. For leaf vegetables (cabbages, lettuce, chard etc) fertilise with a weak liquid organic fertiliser. Dilute it to half the recommendation on the pack, but fertilise every week. For fruiting vegetables (tomatoes, peppers, squash etc) fertilise with the same weak fertiliser you used on your leaves, but do it every two weeks. Fruiting vegetables also need potash - so apply that in the planting hole when you plant, and apply again every four weeks. Potash encourages flowers and fruit so you will get many more tomatoes /squash/pumpkins etc if you use potash. Be careful when you fertilise and know what each one does. Nitrogen encourages green growth at the expense of fruit. If you apply too much nitrogen to a fruiting plant, like a tomato or squash, you will get a beautiful green bush with very few tomatoes or squash.
I am a bit pressed for time today, so I'll come back to this tomorrow and answer some of your questions then too. If you know of a good planting guide for north America, please let me know.
Planting guide for Australia, NZ and UK - choose from the drop down menu at top.
Vegetable planting guide Australia - Eden Seeds.
Articles on various common vegetables.