Dittoing everything TsunamiWombat says. Do NOT play after a stressful day at work/school unless you have enough friends there with you to make a private game- 70% or more of all PuGs end when someone cries, lags, or has their brain suddenly switched out with a dead ferret, and they leave. There is a cushion for this, but one leaver usually leads to another until you're all alone (saying 'stay' only makes them leave faster). Blist remains a joke- a harsher penalty is required to make folks take it seriously.
If you want to try competitive play, definitely learn the lingo and what items work best with which hero- there are several online guides. Creep agro can be important to manage if you're alone, as well as denial. There was also a new hero recently introduced who is pretty good for beginners to use IMO- the Admiral. Good old Kardel Sharpeye, the Sniper, also qualifies, just be ready for folks to complain about your selection regardless. Me, I prefer ranged stunners like Rhasta, Viper and Traxex.