Choose the character and class according to your playstyle. There are stealth, melee, and action-packed heroes along with the classes that combine all the factors in one build;
Make sure to level up the major skills for all Borderland 2 classes;
Choose the right weapons for the corresponding characters. A sniper rifle will better do for Zer0 than for Salvador;
Some classes are better for solo playings like Maya and Zer0 while others like Axton or Krieg are better for teamwork;
If these tips seem insufficient, then consider watching the following video from a hardcore fan who knows everything about Borderlands 2 classes.
https://yaninagames.com/blog/ranking-borderlands-2-classes-from-worst-to-best/