• Obviously source code - patches, as well as completely new files, or entire plugins. I (pawelsalawa), as project owner, am happy to help with this. I can answer questions and provide support for working with SQLiteStudio code as much as I have time.
• Language translations - create new language localization, become maintainer or co-maintainer of certain localization, or just send small fixes for current translation.
• Graphics / art work - in order to create High DPI resolution support for SQLiteStudio, SVG icon set is required. It's very difficult to find suitable SVG icon sets that would fit all requirements of SQLiteStudio - it uses quite numerous icons.
• You can also always support the project by donating: https://sqlitestudio.pl/donate/

There are several ways for that too:

• For source code changes and language translations you can use github fork project and do pull requests
• Send any results of your work to support@sqlitestudio.pl
• You can attach files to comments in a GitHub issue. This can be source code patch, source code file, image file, translation (*.ts) file, or virtually anything else that can be treated as project contribution.
• If you prove yourself as trusted contributor, you may be added as a regular project team member with write access to the repo.

Basically - if you would like to contribute, you can either start by yourself and just share results, or you can talk to me, the project owner and I will help you with whatever you would like to contribute, so it's as smooth and simple for you as possible.