How to become a better player
Guides are almost always a work of personal opinions and thus results may differ.
Pretext
Becoming a better player is something everyone strives for - but what does it mean exactly? Usually, when people say he or she is 'good', they refer to their combat ability - less so on their game-knowledge, or out of combat ability. This doesn't make them any less important however. If you are really good at clearing dungeons - you will help your team immensely, as they will have more resources to spare for use in combat scenarios!
Follow the leader
Before I will list ways on self-improvement, know this. It is vital to follow a leader's commands. Without that, you are actively hindering your team in winning.
Game knowledge
Game knowledge is a difficult concept to grasp. Sure, you could start reading wiki-pages. Maybe try and test out things in-game with an alt-character. But the best thing you can do is talk to others. Many people can give you lots of helpful advise - and they might have already done the testing for you! A lot of information is also not something you can simply 'write-down' - but is either to be experienced for yourself - or told by others.
Medicine
Too many players use First-Aid the first moment they get shot. You are actively hindering your Medic's work, but also yourself. Using First-Aid applies a Bandaged Status effect, that goes down only very slowly - making any further usage of First-Aid heavily penalized. Start using medicine like Bandages, Stimpaks, even Healing Powder, especially if you intend to fall back for a bit and don't need the Perception. This ensures you will still be able to use First-Aid in a critical moment where you need a large amount of HP 'flash-healed'. Also if healing others, try to use medicine as well. If you First-Aid others, they might not notice they received a Bandaged Status effect, until it is too late! You could try unbinding the First-Aid hotkey to try and force yourself to use medicine. Once proficient enough, you could re-add the keybind for First-Aid.
Focus your leader's target
If the leader calls out a target for you to focus - focus it. It is rarely adviseable to ignore his command and instead try to impact the game in a different way. (The more game-knowledge you have, the more you know when you can choose to not follow assist, but in most cases, you will make your team fail if you do not contribute the damage needed to kill a target). This advise goes hand in hand with 'follow your leader's orders'.
Keep your gear in check
Too many people simply use the first gun and armor in a locker they find. The condition of a gun can have a huge impact on its performance. Using garbage guns has so many penalties - that you might even be better off using a weapon not fit for your class, which is however in top condition.
Sure, making sure your weapons are well-kept will take a little bit of time - but it is worth it. Guns in bad conditions will often jam. Have less range. Will cost more Action Points to Shoot and Reload and might have other mali applied to them.
Smokes. Molotovs.
Take them with you. No matter your class, make sure to grab a couple of smokes and some molotovs on your way out to the battlefield. Often times a leader will request a smoke at a specific position - and if you're close to said position - you are expected to smoke it. Not to mention that because most people do NOT take those simple to get utility grenades, the spot will most likely stay unsmoked - and thus make it harder for the squad to come out on top.