I got a new PC and I don't have my old Minecraft worlds how do I get them back?

Do you have a spare flash drive to use? If so then you can transfer them over. Minecraft worlds are not saved to your account, they're bound to your computer.

On the computer that has your old worlds open the File Explorer (where all files, videos, pictures, and what not are stored). In the main bar up top type in %appdata% and hit Enter.

Then go through ".minecraft" > "saves" and it'll take you to a list of folders that contain of all your worlds and everything saved into them. Copy those to a flash drive then transfer them over to the same folder on the new computer. That should do it.

They were probably saved on your old PC.