Begonias should be planted in late Winter/Early Spring.
Find a nice, well drained spot for your tubers - Begonias don't like too much sun, so the hotter your region, the less sun they need! (if planting in pots indoors, ensure that they have access to a bit of light, but not full sun)
Plant the tubers around 3cm below the surface of the soil, ensuring the indented side of the tuber is facing up.
For a full overview of how to plant Begonias visit our Gardeners Guide.