How do you get into Deadlock as a new player?

Learn the parry too, it's F by default. When someone runs at you for a melee, a well timed parry stuns them and you get a free hit. It's the one mechanic that feels useless until it wins you a fight.

Two or three heroes max, yes. Abrams or McGinnis are forgiving while you learn. The thing nobody explains well: when you last-hit a trooper, a soul orb pops out and you have to shoot it to secure the full amount. The enemy can shoot it too to deny you. Getting that right is worth more early than any item build. For items just copy a popular build for your hero from the in-game browser and ignore the rest of the shop for now. Stay in lane until you've got a few items instead of wandering round chasing fights.