The plan is to get it published, yes. The only reason I'm considering registering it now, is because I have shared the idea with other writers and have had others look over my manuscript. I'm 99.9% no one will pull the carpet from underneath me, but it's worth having that protection, right?
Because you have shared your idea with others by having them read your manuscript, you have no protection for your
idea. Ideas can be used by anyone. They have no protection under copyright law.
What you
do have, as FlyingRon indicated, is copyright protection for the specific way you have already
expressed your idea. No one else can use the same words. The copyright protection is automatic.
It is possible to register an incomplete work but most people will wait until a work is completed before registering their work with the US Copyright Office. Registering your work is smart before you present your work to the public. Registration makes you eligible for statutory damages should your work be infringed. You will want to register your manuscript, in other words, before you publish it. The cost to register is $35.
For more information, you can visit the US Copyright Office's official site:
http://www.copyright.gov
This is assuming you are in the US. You failed to mention your state name. Copyright laws can vary in some pretty significant ways from country to country.