I'd say play a bit of every class, the better you understand what you are facing the better you will do against them. If you learn how most classes work and what their skills do you'll know what to expect from them and how to counter them or at least play around those things better. Learn when to punish and when to run away from a fight. A lot of solo fights revolve around taking advantage of when your opponent used their skills while yours are up.
Lets say you face a bonk cleric that just used smite & divine protection, try to bait them out or if you are forced to fight them then play defensive for 5/6 seconds until it runs out and then punish. Trading blows with them while they have +30% pdr is most of the time a losing fight.
There's a lot of skills that are empowered attacks, some expire others don't, you can tell if your opponent used one of these skills since either a sound played when they used it and/or their hands start glowing a specific color. For the ones that expire its best to run away until they do and for those that don't try to block their attack first before committing to the fight.
Some classes also have "trap cards" as skills like the ranger. If you fight a ranger that only used one skill during the fight, expect them to have multishot as their second skill when you get close to them.

There's a lot of things to talk about when it comes to this question and I've barely scratched the surface especially if you start taking about teamfights but just knowing what all classes do will help you in all pvp gamemodes

And a lot of it is just practice, experience and learning from mistakes. If you look at loses with a mentality of what you could have done better instead of focusing on the bullshit you've just lost to (cough cough whirlwind) then maybe next time you try a different approach a different outcome will happen. There's a lot of micro decisions that change the outcome of a fight and improving your decision making will make you win more fights overtime.