When you make a creative work (which includes code), the work is under exclusive copyright by default. Unless you include a license that specifies otherwise, nobody else can copy, distribute, or modify your work without being at risk of take-downs, shake-downs, or litigation. Once the work has other contributors (each a copyright holder), “nobody” starts including you.
Even in the absence of a license file, you may grant some rights in cases where you publish your source code to a site that requires accepting terms of service. For example, if you publish your source code in a public repository on GitHub, you have accepted the Terms of Service, by which you allow others to view and fork your repository. Others may not need your permission if limitations and exceptions to copyright apply to their particular situation. Neither site terms nor jurisdiction-specific copyright limitations are sufficient for the kinds of collaboration that people usually seek on a public code host, such as experimentation, modification, and sharing as fostered by an open source license.
You don’t have to do anything to not offer a license. You may however wish to add a copyright notice and statement that you are not offering any license in a prominent place (e.g., your project’s README) so that users don’t assume you made an oversight. If you’re going to accept others’ contributions to your non-licensed project, you may wish to explore with your lawyer adding a contributor agreement to your project so that you maintain copyright permission from contributors, even though you’re not granting the same.
Disallowing use of your code might not be what you intend by “no license.” An open-source license allows reuse of your code while retaining copyright. If your goal is to completely opt-out of copyright restrictions, try a public domain dedication.
If you find software that doesn’t have a license, that generally means you have no permission from the creators of the software to use, modify, or share the software. Although a code host such as GitHub may allow you to view and fork the code, this does not imply that you are permitted to use, modify, or share the software for any purpose.
Ask the maintainers nicely to add a license. Unless the software includes strong indications to the contrary, lack of a license is probably an oversight. If the software is hosted on a site like GitHub, open an issue requesting a license and include a link to this site. If you’re bold and it’s fairly obvious what license is most appropriate, open a pull request to add a license – see “suggest this license” in the sidebar of the page for each license on this site (e.g., MIT).
Don’t use the software. Find or create an alternative that is under an open source license.
Negotiate a private license. Bring your lawyer.