Disclaimer: I am not a lawyer, the below is simply my understanding of OSS licensing. If I'm wrong, I'd appreciate any corrections.
- Clearly state (on your project website, in source code, documentation, splash screen, about dialog, etc.) that the project is licensed under whichever license you select.
- Considering the nature of open source, if a portion of your project is used in a different project owned by someone else (e.g. some useful utility class), having the "basics" of the license as part of your source code's header text makes it clear which license governs the use of that specific source file, and who owns the copyright on that portion of code in the project as a whole.
- Yes, unless they have supplied you with documentation stating that they have transferred copyright to you (I believe some larger projects require contributors to do this?).
For an example of applying an OSS license to a project, see http://www.gnu.org/licenses/gpl-howto.html
