Most gardeners prefer to start their garden using seeds. You can buy a variety of different cultivars of the same plant.
Thanks to modern day horticulture research, most seeds are hybrids with features such as virus and fungi resistance. The plants tend to grow fast and produce healthy vegetables.
With seeds you will save money and also be in a position to buy a better selection.
Be sure to read the instruction on the seed packet.
Some seeds (ex lettuce) would need to be sown within a year while some (ex beans) can be retained for a several years.
There would recommendations that some seeds would be better off if grown indoors, while transplanting the seedling later into the garden.
Keep the seeds in a location which is free from moisture and is quite cool.
Two pointers to keep in mind while planning on when to sow the seeds would be
Determine the last frost date for the spring. Usually it would towards mid to late spring. After this day the soil should start warming up and you can plant your seeds directly on the soil.
If you are planting cool season plants you can start of little earlier than the last frost date, but if you are working with warm season plants be sure to plant them only when the soil is warm.
A few specific examples, on planting times for certain vegetables, are given below
Lettuce:
These can be planted 4 to 5 weeks before the last frost day, while the soil is still cold.
Onions:
Can be planted 4 to 6 weeks before the last frost day
Broccoli:
Can be planted a couple of weeks before the last frost day. Make sure there is no ice on the ground or any indication of snow.
Eggplant:
Can be planted early, 6 to 8 weeks before the last frost date
Peppers:
Highly tolerant to cold, can be planted 8 to 12 weeks before the last frost date
Cucumbers:
Can be planted a couple of weeks before the last frost date.
Just remember that the above guidelines hold true only if you are planning to plant a seedling or a transplants in your garden.
If you are planning on sowing the seeds directly, then the best advice would be to do it once the soil is warm.
Cold soggy soil is not congenial towards the growth of a seed. Most seed would just rot when placed inside cold damp soil.