I'll bite :)
- Stick to the basics to start: Mattermost/Slack for messaging, Github for issues/work, standard tooling (I imagine you might be B2B SaaS? So maybe Node or Python). There's a small chance you're building something that's actually "new" - so just stick with what is tried and true.
- Have an onboarding doc! Even if you don't stick with it, or even if it doesn't make a lot of sense. When do people get paid? Who to go to for HR-like questions? Is there a virtual happy hour on Fridays? How should one communicate on Github/Slack? How do I get my project setup with all proper dependencies, etc. This saves a ton of "Oops, we never actually clarified this" down the road.
- This doc is where you help clarify company culture (#NoSmartAssholes) and stuff like this.
- This is also where you can set expectations on the role, and what's expected.
- Have work ready to go. It will only take a decent programmer a few hours to get setup (obviously depending on the project, but you mentioned you're small so it's probably just a repo you can clone). Use those Labels on Github issues liberally. Have a slate of "Good first issues" ready to go that can introduce the new hire to all/most areas of the codebase gently :).
- Lastly, vibe is maybe a little more important than technicals at first? Of course there's work to be done, and it needs to be done correctly, but on a really small team being able to have an actual discussion about "What did you do this weekend?" is maybe a bit more important than "Proper tests were not added for this new feature".
TLDR: Use the same comms platforms as everyone else, have a decent onboarding doc, have some work already triaged and ready to go, focus on good vibes and the technical stuff will come :)