How can I determine whether a GitHub repository is suitable for first-time contributors?

First start doing review of existing pull request (PR). If you can read new code and understand it, then probably you can create your own PR.

Trying and see the result is also good approach. Do not be discourage, there are some projects where people are happy to see new contributors and there are some when you will be seen as invader.

Note most good projects have instructions for contributors.

The main indicator I would consider greatest blocker to be a contributor: Do you know what test are (unit/functional/....)? Can you write them? Do you know what coding standard is?If you answer to any of this is "No" then you have to catch up, before trying participate.

On other hand this works into opposite side too. If you see a project without tests and properly configured CI, then most probably you do not want to be part of it.

Usually you don't pick random repos for contribution, but you contribute to projects you are interested in as a user (tools you use, libraries your own project depend on). Then you have pretty good idea what it is supposed to do and how it is supposed to do that. Then when you find a bug or have an idea for an improvement you can raise an issue and/or submit a pull request implementing it. This way it is pretty much up to you what would be th complexity of your contribution.

You cannot just look at a project and know if you can contribute meaningfully. You have to read the code, engage with the community, etc.

But does it even matter? You should engage with / contribute to projects that you care about - and if your skills are not up to par yet, they soon will be after you start talking to people.

Contribute to what you care about, not what you presume will gain you something.

Do you understand the code?
Do you see what/where it goes wrong?
Can you think of an improvement, and, better yet, can you document it?

If so, go ahead. If not, don't.

"If you see a project without tests and properly configured CI, then most probably you do not want to be part of it." - Or maybe that's exactly what you want, since that gives you an easy way in by fixing CI or contributing tests... Could be a great way to start.

You can hangout on reddit/C_Programming (and similar programming subs), and very often new (and experienced) coders will post links to their github projects. In time, you'll see something from a newb that interests you, and then just ask. That's the easy way, imo.

Recommendation: Also read through the project forum, bug trackers, etc... to get a feel for the development community. You may have the skills necessary to contribute, but if they project is overrun by holes, will you want to contribute?