Make sure that your system is up-to-date:
sudo apt-get update
and
sudo apt-get upgrade
For Ubuntu, install following dependencies:
sudo apt-get install erlang libncurses5-dev libssl-dev unixodbc-dev g++ git erlang-base-hipe
Next, download Aeternity Testnet. Optionally you can run next steps with a non-root user, for better security.
git clone https://github.com/aeternity/testnet.git
Now you can go into the directory, and compile the aeternity testnet.
cd testnet/
sh install.sh
Installation should be done. Now you can run your node.