Sweetie, you can *not* become a mermaid, or any other creature on this plane. If you want to be a mermaid so badly, go to the astral plane and you can be.
It is a simple matter of deconstructing the body, then reconstructing it. Only thing is...you can't do that currently. You don't have the energy requirements to do it. Nor does anyone else that I know of. You'd have to change the body, cell by cell in order to change it. To do this...you'd need to fully understand how the body works as well and chemistry to understand what to change. Lol. Then you have to actually change it, which would be a painful process to undergo...you can possibly die. Lol.
Still sound like a fun idea to do when you aren't yet grown up?