Copyright, trademark, and patents are three very different things with very different laws concerning each. You seem to be confused on the basics of these matters you should do some research; here is a document explaining some of the US copyright system.

In general:

• Copyright protects a specific expressions of an idea.
• Patents protect inventions; generally physical.
  • Abstract patents are generally invalid, see Alice v Bank
• Trademark protects business names, logos, etc.

Copyright attaches when the work is fixated, i.e. when you write it down, record it as a file on a storage device, etc. You don’t need to register a copyright to own it; you need to register your copyright if you want to sue someone and be eligible for statutory damages. Actual damages can be awarded even without registration (n.b. I Am Not A Lawyer (IANAL.))

can someone else just take my plugin and copyright it?

Legally no, could they register it anyway, possibly.

can someone trademark the name of my plugin

I don’t know much about trademark law, if they are using the mark then probably; i.e. they happen to have a similar product already that uses the name. The test, if i recall correctly, is about market confusion, i.e. could someone confuse your product for mine. Also, trademarks have to go through a ‘comment’ period where interested parties can object to your mark.

What if they file for a provisional patent on my work? Can they do that?

Legally no, since software patents are abstract and are rarely valid, but invalid patents are granted all the time. Most software patents wouldn’t hold up in court but the cost of litigation allows invalid patents to be wielded as a threat, again IANAL!

You establish ownership by asserting copyright.

I can easily do this for a $1000 fee. Or you could do it yourself by inserting a comment:

// Copyright Your Name Here , 24/04/2015, all rights reserved

at the top of your source code.