You can try doing it from a blackberry- it has seeds in it. But it would be easier to take a root or stem cutting instead. Dig up a healthy spring plant and pot it up. It can be done- I've seen people growing blackberries in whiskey barrels. I don't know if they were wild ones or not.

With very few exceptions, fruit from a wild blackberry is not going to be the same quality as a fruit from a cultivated variety of blackberry. They tend to be much smaller and seedier and often lack the degree of flavor cultivated forms of blackberries do.

If I was going to bother with growing a blackberry from seed - a tedious process at best - I sure would not want it to be a wild form!! Same with starting from a root or cutting. Get hold of a proper cultivated variety that you like and put your effort into that. Wild ones are just not worth the effort!

If you do find a wild blackberry with good fruit use cuttings or layering to reproduce an identical plant. Using seed could produce something quite different. fwiw I gather and freeze many containers of wild blackberres every year and they are very juicy and very tasty and free. But I would not bother growing them.

You mention "it" and "a piece of fruit"; note that each fruit contains dozens of seeds.

If you want to experiment and have a square foot of bare soil, just mash one over-ripe berry and spread it around on the soil; next spring some seeds will likely sprout. The leaves are distinctive and not readily confused with other weeds. I get seedlings every spring from the previous year's wayward and underfoot fruit.