In addition to @pharesim's correct comment about 1/21 blocks being mined by proof of work, anyone can indeed be voted as a witness. 1/21 blocks are signed by a backup witness with frequency proportional to the number of votes. A backup witness is one that doesn't make the cut to the top 19. You can just vote for yourself, set up a witness node, and occasionally (very occasionally if your stake is small) sign a block and get the rewards. If others vote for you, you will sign more blocks.
The top 19 are special because they get to sign a block every round, so 19/21 of all the blocks on the chain are signed the top 19 (1/21 each).
Thank you for clarifying that. I still have to finish reading the whitepaper.