Below you find a download link for a calendar with the best dates for sowing your seeds indoor as well as outdoor. The calendar also shows the flowering period and harvesting period, which of course depends on your sowing and planting time.
Many crops can be sown earlier indoors than outdoors. Especially for cool regions, with a shorter growing season, most seeds should be started indoors, as they will have a better start and you can start sowing earlier. In warm regions with longer growing seasons, more seeds can be started outdoors.
Make sure to only start planting most of your crops outside once the risk for frost in Spring is over.