If you want to set up a testnet node, here are the general steps (this is how you do it in general, but it helps to know how to deploy your own):

https://developers.steem.io/tutorials-recipes/setting-up-a-testnet

Once that's working, here's how to wire your testnet node into the official testnet (you can skip right to these steps if all you want is to connect your node to the official testnet):

https://developers.steem.io/quickstart/#quickstart-testnet

Update: It appears that Ubuntu 16.04 LTS is no longer supported for these steps (wrong version of boost). Maybe boost-1.65.1 can be installed on Ubuntu 16.04 if you bypass the package manager. But for these steps, at least at the moment, it is recommended to switch to Ubuntu 18.