1. Shoot rockets 2. ??? 3. Profit.
The soldier is one of the most potent offensive classes that TF2 has to offer. The rocket launcher is excellent for taking out heavies, sentry placements and anyone running up to your at close to medium range. The critical rockets are perfect for dispatching pesky medics that are healing people that are shooting you. The rocket jump assists in your mobility, gaining you access to parts of the map that other classes have to run further to get to. They’re also the best class at taking out sentry guns from afar. Pair yourself with a medic, and you’ll be able to make scrap out of any gun that isn’t being actively repaired by an engineer.
The shotgun is a tool to use when you run out of rockets and have someone breathing down your neck. They’re also good for tagging fast classes like pyros and scouts that you can’t quite get a handle on. Don’t underestimate the shovel, either - if you’re right in front of someone, switch to it quick to dig a hole in their skull.
The way to get good at playing the soldier is to master when to reload. You shouldn’t be reloading after every rocket you fire, but rather whenever you detect a lull in the battle and can duck behind cover. Don’t launch all four missiles in rapid succession unless you can be confident that all of them will hit something. Try to go for direct hits whenever possible - the splash damage of the rockets is relatively low unless it’s a critical. If you’re going to be ubercharged, make sure you have all four rockets and a good plan of where to send them. Dodge in and out of cover as you launch rockets at stationary targets. No heavy should be able to withstand you.
If a pyro is right on top of you, don’t panic. Shoot a rocket or two are your feet - you won’t take as much damage, and the ensuing blast will likely splatter them all over the hallway. The limited ammo capacity can be a problem, so be sure to visit a dispenser after bringing down a heavy rain of rockets on your enemies. The soldier is also a relatively tough character with a high health point total, meaning that you get significant benefits from pairing with a medic, at least for a little while.