Get a command block in minecraft - how?

on bedrock it's /give @s command_block. heads up, turning cheats on there switches achievements off for that world for good.

You can't craft it, you have to give it to yourself. Cheats need to be on, then type /give @p command_block in chat. If the world was made without cheats, open it to LAN and switch Allow Cheats on, that works until you close the world. Also you need to be in creative to actually edit what's inside it. On a server you need op, and enable-command-block has to be true in server.properties or it does nothing.