This is hugely important: despite great weapons and accs, if you do not understand the game's mechanics, even well geared players can find the game tricky.
Learn the game mechanics:
Enemies - types, their attacks, timing, scripted movement/placement/reactions.
Aggro - how to draw it, how to avoid it - when for both.
Support - if your character is a support type, learn how best to support the team (barrier, well placed wall of fire, winter's grasp etc).
Combos - learn how to inflict status effects with your powers, and in conjuction with other players (ie, paralyze, slow, weaken).
Characters - learn their strengths and weaknesses, and timing for using powers in what order to maximize damage output.
Map cover usage - learn the maps and how to use cover to minimize damage taken while whittling down enemies (one of the biggest fails is often fighting out in the open, taking damage from multiple angles). also here, learning to draw enemies around a corner or through a choke point in order to control their movement and number of enemies attacking at any given time.
When to, when not to - a great example is reviving teammates. sometimes it is safe to revive. other times, if enemies are too close (and/or mucho ranged atacks), you will also be downed before being able to revive. at times it is best to clutch the wave (solo) rather play romantic hero and die in the process.
Gold! - as a new player it is super important to collect gold from vases and chests to buy chests in the store. mic up and tell other players you want to collect all the gold since you are new and need it. some players who have the good stuff will often skip gold, but they have probably forgotten how painful it can be to be a new player with the lowest weapons and accessories.
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These are some things to consider, aside from weps, accs, and promotions. I have seen many a player with good gear, but they lack the strategic understandings mentioned above and often faceplant (and so say 'this character sucks'). It's not the character, but the player who sucks. Understanding your characters and how to best to utilize them against enemy units is the key (granted, great weps n accs certainly help, but are not the end all be all).
All the best to you OP, and welcome to the DAIMP community.