Apache
Blackberry in the US

You're usually looking at planting blackberries as bare root canes or potted plants, not often from seed to be honest, as it takes a long time. If you do start from seed, give them a good long chill first, then sow them shallowly, about a quarter-inch deep, in early spring after your last frost, or in late fall depending on your USDA zone. For most folks, this means March or April, but check your local frost dates; my GrowMore CookMore app gives you the proper timing for your specific zone. They need a bit of warmth to get going, around 65-70°F (18-21°C), and will take a full year, maybe more, to give you fruit.

Once your young plants are growing well, or if you've bought canes, get them hardened off properly. Wait until all danger of frost has passed in your area before planting them out into the garden. Give them plenty of space, about 3-5 feet (1-1.5 meters) between plants, and make sure they have a good strong support system, like a trellis or wires, because those canes can get long and heavy with fruit, you know what I mean. They're not really for containers, to be honest, they just get too big.