You have two options:
- Write your patch and submit it as a pull request on GitHub.
- Post about it on the forum.
All contributions are accepted as dual-licenced with both the Angband and GPLv2 licences.
There are contribution guidelines in CONTRIBUTING.md in the top level directory of the source code.
If the patch is a bugfix, then you can reasonably expect it to be integrated into the development tree. If it’s more involved, and the feature is not one that the next version is planned to have, the patch may go through several reviews before being incorporated. It may also just be unsuitable for Angband – in which case, please don’t take rejection badly; you may just be better off writing a variant.
Non-code activities are different. Documentation can be written on the wiki, or if you’re a graphics designer (and they’re always welcome) then please talk on the mailing list about your work.