Torbjorn is a tricky hero because so much of his effectiveness is reliant on your ability to place his turret somewhere where it won't be killed. Of all the heroes in Overwatch, Torbjorn is likely to be the least used because of this. Unlike many, he will only be useful in certain situations—like playing Escort or Assault game modes on the defensive team. Otherwise, he's just not all that viable. That said, there are moments when he can really shine, which we'll get into below.
Rivet Gun [LMB] - Fires molten projectiles with a slight arc that deal moderate damage with a fairly quick speed of travel. The alternate fire mode is akin to a shotgun, firing a short-ranged blast that can be devastating for anyone who gets too close.
Forge Hammer [LMB] - One of the only heroes that has a secondary weapon, Torbjorn's Forge Hammer is primarily used for constructing and repairing turrets by hitting them with it, but it can also be used to whack someone in the skull if they get close.
Armor Pack [E] - By collecting scrap from fallen heroes, Torbjorn is able to convert it into armor packs that he drops on the ground and can be picked up by either himself or his teammates. You can't stack the armor earned from each pack, so it's best to spread it around and make sure everyone gets one.
Build Turret [LSHIFT] - Allows Torbjorn to select where he wants to build his turret. We'll get into the dynamics of the turret in the General Strategy section below.
Molten Core [Q] - Molten Core ultimately acts like a massive buff to all of Torbjorn's abilities. He gains a significant chunk of armor, his repair and build rates are increased, the firerate of his Rivert Gun is increased, and, most importantly, his turret is upgraded to level 3, significantly increasing its damage.
Scrap Collector [Passive] - As heroes die, Torbjorn can collect scrap from their bodies that he can use to craft armor packs for pickup. Each hero drops 25 scrap when they die, and each pack requires 50. Torbjorn can only hold 200 scrap, so be sure to craft armor often.
Torbjorn Strategy Tips
As I mentioned, Torbjorn is a tricky hero to play because he's only useful in a handful of situations compared to similar defensive heroes like Bastion that have a bit more viability. That said, when playing on defense, Torbjorn can be a huge asset to the team if he can get his turret built and upgraded.
When you first place your turret it will build quickly as level 1, meaning it has less health and fires half as fast as the level 2 variant. Fortunately, Torbjorn can upgrade the turret just by whacking it with his Forge Hammer a few times, which you should always do. The turret itself is surprisingly effective at laying down fire and its range and tracking speed will make it formidable for any hero unfortunate to cross its sights.
The problem remains that once enemies are aware of where your turret is, you're going to be constantly fighting to stop them from taking it out—which can be really frustrating if playing in public matches and your team isn't assisting you. When you're turret takes damage, just give it a few good whacks to get it back into fighting shape.
Positioning here is extremely important, but one thing a lot of people fail to consider is also compensating for where Torbjorn is going to have to stand in order to repair the turret. The rate at which you can repair your turret is actually fast enough that only a concentrated attack will bring it down, meaning smart enemies will target Torbjorn first and then kill the turret. When placing it, always make sure you have room to repair it without putting yourself in danger.
Finally, Molten Core can feel like an extremely underwhelming ultimate because so much of its potential is wrapped up in the upgrade your turret gets to level 3. If it's killed off or not in a position to eviscerate the enemy team, all you're really getting out of Molten Core is the extra armor and increased rate of fire for your Rivet Gun, hardly the kind of things that win a fight.