I hired my first employee and hes starting on Monday! Can you give me some advices on how to onboard him smoothly and be a good boss?

Have all his accounts ready to go, software installed and instructions for getting into it.

Have a beer ready

Be a good fucking employer.
Don't be the kind that has unrealistic expectations and give them enough time to get used to everything.

Also best of luck in your business!

Just be yourself.

The weirdest thing is when someone tries do everything right...

And actually behaves like chucky the murder doll while trying to over achieve.

@IntrusionCM well said , thanks!

Have time to get him into the project.

My best start at a project was when they actually had a handbook which described every micro service and how the data flow was. Then it had three test questions to actually fix some prepared "bugs" in the code.

It helps in two ways. You can learn how the employee thinks and acts while they learn the basics of the project.

Also I see others say preinstall software. There is nothing I hate more at the beginning of a project is not being able to set things up my way.
You should document how to setup the project, but in my opinion setting up your own PC is important, I want the shortcuts I'm used to not what my boss or colleague is used to.

The importance of deliverables vastly exceeds the importance of hours and appearances. Be lax on basically everything but quality and performance. If they want to work 10pm to 3am but finish their projects on time, who cares? If playing quake helps them think, who cares? Also remember that shit happens and not everything will go according to plan.

@Root It depends on what kind of project that is. In my case its a gameserver and since wel be doing lots of pair programming in the beginning its impossible to allow him to have whatever schedule he wants.

The most important elements of any onboarding process is to show them where the toilets are and where the coffee is.

I always start with that, with visitors at the office too! And at home I tell where to find the toilet and (usually) beer.