Which is the best licence for my web application. GPL

Hello,

I have created a CMS application and of course I would like, that people are using it - download it and try it out.

I just wonder what could be the best licence I could publish it under?

I thought of a GPL?

Can, I change the licence type again in a later version?

I would people to use it for the following reasons:

- It helps me to find out, if this new CMS has an advantage over other CMSes.
- It helps me debugging (It should be bug free as much as possible but I am now it the stage of testing.)
- Developers can add and remove from the code, but not be able to sell it as their own.

- I would like to know, what I could add to it next to make the CMS attractive to use.
- Developers can use it for their own projects free of charge and shall be encourage to write modules for it.
- However, I would like to remain the owner of it.
- Developers will be able to charge their own clients, for setup, templates, etc, but not the CMS.
- I would like to protect my project.

How do I include the licence text and "I agree" button into the web application,so that it is binding?


Thank you for your help!


C.