Ok, this is the guide I first used when setting up my Pi.
https://www.raspberrypi.org/help/noobs-setup/
Then used the wiki guide to install the other stuff needed for Homebridge to run.
https://github.com/nfarina/homebridge/wiki/Running-HomeBridge-on-a-Raspberry-Pi
Then install Homebridge following the ReadMe guide here.
https://github.com/nfarina/homebridge/blob/master/README.md
You'll then need to install the plugins you need for the devices you want to control.
https://www.npmjs.com/browse/keyword/homebridge-plugin
And then update your config.json file with the necessary info.
You can use this sample, just remove the '-sample' part from the file name and update with your devices. The plugins on the NPM website should tell you what to write in the config.json file to get them to work.
https://github.com/nfarina/homebridge/blob/master/config-sample.json
You can check your config.json file is correct by copying and pasting into this website.
http://jsonlint.com
You should be able to run Homebridge now and add it as an accessory in a HomeKit enabled app such as Insteon+ or Elegato Eve.
You might also want to run a VNC server on your Rpi so you can check up on it without having to connect a monitor/keyboard etc. if Homebridge crashes or you want to make some changes to your config.json file.
https://www.raspberrypi.org/documentation/remote-access/vnc/
Ok, that does seem like a lot to do! But if you read through the info in the links a few times and dedicate an hour to get it all done you should be fine!
Good luck!