There should be some worthwhile tutorial videos on youtube that you might want to check out. Anyway, aim at their feet for splash damage, jump while firing or use higher ground to spread the damage. Work on rocket jumping, it gives you a significant advantage once you can pull it off reliably. Fire a rocket at your feet while jumping (or crouch jumping) to pull it off, preferably have some momentum beforehand. It not only aids you in killing enemies but it can work quite well defensively when you need to retreat quickly. Rocket jumping also lets you reach areas most other classes couldn't. You can juggle enemies mid-air with your Rocket Launcher too, you can send them in the air with the splash damage at their feet and follow it up with another rocket while they're still in the air.
If you get ubered and need to take out an engineer and his sentry, kill the engy first. If you've used all four rockets, switch to the shotgun to finish the job rather than reload the rocket launcher. You can also take down sentries from far away by spamming rockets at it. If a sentry is right around the corner, aim at the ground close to where it is and finish it off with splash damage.
It's fairly easy to become competent at soldier but being able to do more than shoot the floor separates you from the pack.
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