20,000 Light Years Into Space

As a rule of thumb, you need one steam maker per city tech level. But you'll want more than that to survive attacks and damage from the environment.
Don't start upgrading your city straight away. Get more steam first.
Be on the lookout for pipes that have been broken by attacks. Repair them immediately. Always try to keep some Work Units available for emergency repairs.
Things that aren't connected to the city don't get repaired, upgraded or built.
Don't make your pipes too long. Have plenty of nodes to facilitate branching. Nodes cost more time to build, but they're really hard to destroy and they permit your network to branch.
Having more than one route between two points is always good. It makes the link more resilient to attack, and it gives a lower resistance, so more steam can flow.
Things that are connected to the city are repaired, upgraded or built in a breadth-first order. That means that the things that are closest to the city (on the network) will always be done first. You can use this property to set up a build order.
There's no way to prevent attacks from the environment. You must defend yourself by improving your network.