Soldier 76 is the Mario of Overwatch. Hes decent for just about anything. His rifle does decent mid-to-long-range damage, hes got rockets with knockback, and he can even heal. Oh, and his ultimate ability is basically an aimbot.
Reinhardt is Overwatchs most straightforward tank. Hes got a big ol hammer, lots of armor, and a holographic shield allies can shoot through but enemies cant. Many strategies revolve around a frontline with Reinhardt at the head. He can also charge enemies and pin them against or surfaces, and his ult knocks down everybody in its path, briefly stunning them.
Among Overwatchs supports, Mercy is the most straightforward healer. The Caduceus Staff can either heal allies or boost their damage output. She can also quickly fly toward allies, and her ult resurrects fallen friends. However, Mercy is extremely weak in terms of offense and defense. Always stay behind tanks and other beefy allies.
Mei is a very effective Defense hero, and despite her innocent appearance, she is an absolute terror. She can freeze enemies solid with her Frost Beam and then knock their brains out with a giant icicle blast. Her Ice Wall is great for giving allies cover or obscuring enemies shots. Her ult slows and freezes everyone in a fairly wide radius. Oh, and if shes hurting, she can freeze herself, becoming temporarily invincible and recovering health.
As for general tips:
-If you're getting a solid-gold ass beating, try another character. If youre not sure who to go for, ask your team what they need help with at the moment. If one of your teammates decides to be a dick, because how dare you not know everything about the game, kindly remind them that youre just trying to help, because thats how you win in Overwatch. Odds are, your other teammates will have your back on that one.
-Communicate. Let people know where youre going, what your plan is. If the other team has you stuck at a chokepoint, suggest a coordinated assault or a specific strategy instead of individually banging your head against an impenetrable defensive wall.
-Don't be afraid to lose. Dont lose hope, just because things arent going your way. Overwatch is designed to have the spoils of defeat being not too far from victories, as there are a lot of cool, awesome things that can happen during the match.