voice chat is fireteam only always
things to do when you first hit 20 are : bounties from the robot guy, to build vanguard rep, crucible rep (pvp), queens rep for next 2 weeks. you can do public events on earth to farm vanguard rep, and just endlessly PVP to build pvp rep. finally doing the loot treasure cave for hours and hours is the fastest way to gear up, but the most boring. not much point in doing strikes, at all - get rep and tokens faster with public events.
every time you hit a new rep rank (cryptatch, vanguard, pvp) after rank 3 you get a gift of blue/purple engrams. also you wanna collect as many strange coins and mote of lights as you can (engrams and pvp are good sources), xur shows up on weekends and sells exotics for them