Welcome to OW
You don't really need voice chat but it's strongly recomended. OW is a team game and the more organized you are, the more you will win.
There are probably counter-guides on the internet, I haven't checked myself. But if you just play lots of Quick Play, you will eventually learn. Mostly, think logically. A Pharah flies most of the game, so a Reaper or a Junkrat are weak counters for Pharah as their attacks are not very long ranged. Instead, go for a guy with long, straight shooting bullets, like Soldier 76 or Mccree. A sniper is best countered by someone who can get to them fast, before they get shot, like a Winston or a Genji. You will learn eventually what to play and what to avoid playing etc.
You are expected to know at least one of each role (dps, tank, healer) for competitive. The common setup is 2 dps, 2 heal, 2 tank and is generally something to follow at most times. If the 2 dps and 2 heal slots are taken, you are expected to know a tank. You CAN ask to play healer and dps instead, but if they don't want to switch, the blame will fall on you if you don't pick a tank and perform with it. It's a flawfull system but that's how it works. You don't have to focus on learning both Offense and Defense heroes. Overall, all heroes in Defense are pretty undertuned and often recieve a lot of hate. Unless you are a GOD, as in extremely good with some of them, stick with the Offense heroes.
My best advice is to start with Soldier 76, Reinhart and Mercy, they are probably the easiest of each category to play decent with in the beginning, then move to more advanced heroes. But all up to you, hope you enjoy the game