First step is download this repository

You'll need git first and foremost. Open the Terminal app type:

brew install git

Then change to a directory where you want to download this app. Maybe something like, /Users/your_name/Projects/:

cd /Users/your_name
mkdir Projects
git clone https://github.com/HunterMeyer/Circle
cd Circle

For the next steps you'll want to keep Terminal open and make sure you're in the Circle directory.

Next step is to install React Native and probably Android Studio and XCode

It's easier to read React-Native's documentation to figure this out.

npm install