install erlang version 18 or higher. You may need to install from source http://www.erlang.org/downloads
Or use Homebrew:
brew install erlang
Use git to download the software, then go into the testnet directory
git clone https://github.com/aeternity/testnet.git
cd testnet
mkdir data blocks
Install the dependencies:
sh install.sh
Now you can run your node with sh start.sh