Dear Newbie,
For continuous harvest of something like carrots, you need to plant seeds every couple of weeks. One seed will produce one carrot. Some lettuces, you can keep cutting the outer leaves and the plant will continue to make more until it gets too hot outside.
My Blue Lake bush beans produce, then take a rest and then produce again. But I usually plant some more seed a few weeks after the first sowing to have beans during the first batch's rest period.
Seeds that you only get one harvest from one seed are beets, carrots, turnips, rutabaga, parsnip. Things like tomatoes, peppers, eggplant keep growing and flowering, so you only plant once a season.
Leafy greens like kale, chard, mustard will keep producing new leaves as you harvest the outer leaves.
Some gardeners will plant cucumber seeds every couple weeks, but I've had good luck making only two plantings a couple weeks apart. They just keep coming.
Cabbage is also a one seed, one plant type.
The right type of broccoli will produce side shoots after the main head is cut off. Last year I planted the hybrid 'Packman' and it kept producing all summer and even into December.
Hope this helps.
Keski

Keski is right on. If you want a continuous harvest with carrots, you have to sow new seeds every few weeks during the planting season.
Swiss chard is another vegetable that keeps on giving - cut the outer leaves and more keep growing. Not forever, of course, but for a long time. I don't know where you live, but there is probably a Master Gardener program in the area that can give you specific advice about what and when to plant for your climate.

Here is a link that might be useful: What's Growing On?

There is a great little book called 'Week-by-Week Vegetable Gardening' by a father-daughter team author Kujawski. Not too expensive, lots of tips. You figure out your zone and last spring frost date and fill them in and then you have the dates and schedule for planting and starting seeds.
Keski