Planning which seed varieties to grow, starts by checking your existing stock of seeds. Even though the packets may have been opened, provided they have been stored in cool, dry conditions, most seeds should be good for another season. More than that, you are chancing your arm!  However, parsnip seed should be fresh each year.
Deciding which new varieties of vegetables to grow, is a past-time for a Winter’s night, spent looking through Seed Catalogues. Remember, some Seed Catalogues are now  on-line.
For winter and spring vegetables you must start with suitable hardy varieties of seed, which should  be sown and planted out at the time suggested on the seed packet, making an allowance for a shorter growing season, if you are North of Manchester.  Brassicas require to be always protected from the birds with netting, while broad beans and peas need fine mesh protection from birds and mice when young. Try starting broad beans and peas in pots in February in the  greenhouse or cold frame, with protection from mice and slugs, and then plant out under cloches for early crops.
For continuity of cropping, try sowing one half of the row of a vegetable in April and then again in July. Particularly useful for salad crops, peas and beans. Also try sowing varieties that have different cropping times to spread cropping.  F1 varieties tend to crop all together.
The shorter growing season in Scotland and the North of England may require the sowing of seed varieties that have a shorter period from planting out till harvest. Very few seed catalogues give this vital information. It is particularly  important when you intend to grow crops such as Sweet corn, Winter Squash, Butternut Squash, and Tomatoes outdoors, and for Greenhouse crops such as Peppers, Aubergines, Cucumbers, Melons and Water Melons, and Tomatoes for the earliest crop.
When sowing in heat is recommended, a simple window sill propagator in a sunny window should be adequate. The seedlings will need as much light as possible to prevent them growing leggy. If you are using a heated propagator, then you should reduce the heat to room temperature, once the seeds have germinated and increase the ventilation.
The young seedlings can be moved to an unheated greenhouse or a cold frame once the danger of severe frost is over, say mid-April. The increased light levels and cooler temperatures will give stronger, less “sappy” growth.  Extra protection with bubble wrap is a good idea. See  “Organisation of Greenhouse”.
Potting on of the seedlings may be necessary if you have sown your seeds all together in the same pot. To cut down on “potting on” of seedlings and save time, disturbance and damage to the seedlings, I favour sowing the seeds more or less separately in individual cells. If it is intended to plant out a group of seedlings together, then sow several seeds in each cell, as shown in the “window sill propagator” photo.
Allowing the seedlings to dry out is the most common cause of failure following  germination of seeds. As they still have tiny roots, they cannot cope with any interruption of moisture supply. The use of capillary matting and a water reservoir, as in the photo, will help to prevent disaster.
Damping off of seedlings or rotting of seeds is usually caused by not enough ventilation, and/or saturated compost. Compost must be able to drain, so that the seedlings are not drowning!
Do not let slugs and snails eat your young seedlings! In a protected location, such as a greenhouse or cold frame, it is alright to use slug pellets.
Do not plant out tender varieties too early. Tropical or Semi-tropical varieties, such as Sweet corm, Courgettes, Squashes, Tomatoes, Peppers absolutely  hate it and frequently  die off if they get chilled by a cold wind at night. You would not like to spend the night outside in April!!