Typically the best time of year to plant your seeds is right when the cold weather of Winter is starting to decrease and it is becoming Spring. This will give your seeds the optimum time to germinate and grow into healthy seedlings.
Watch this video on how to start seeds:
Now of course this will depend on where you live, as sometimes people in the Southern part of the US will be able to plant their seeds in the fall and as the Spring and Summer are too hot to grow plants outdoors.
An exception to this is growing peppers as they prefer to be in the hot and arid environment of the Southwest.
If you live further North, you will want to time your planting of seeds closer to Spring when the weather starts to warm up. All plants require sunlight, but like all good things – too much sun will hurt any plant except some desert plants such as cactus and other succulents.
You will need to determine what grows best for your environment, and then make your planting decisions based upon your locale.