As there is so much variability in what spellcasters can do, the most important thing for them to be is flexible. As a player, this means keeping tabs on what is happening in-game, planning ahead, and ideally having a backup plan or two. Having good knowledge of your character’s spells and abilities is paramount, and makes for a good player in general.
To illustrate the importance of flexibility in combat, imagine a scenario where your party is battling a group of cultists, you’re well outnumbered and looking pretty beaten up. Your Wizard is geared up to cast Magic Missile; not risking an area of effect spell as your party’s Monk and Paladin are in the fray. However, the Monk is getting low on hit points and decides to disengage after getting a couple of hits in. With them at a safe distance, you determine that using your only 3rd-level spell slot is now worth it, as you can position a Fireball so that it catches all of the cultists. You know that your Paladin has advantage on the saving throw and will likely only take half damage. The Fireball rids you of many opponents and makes dispatching of those remaining an easy task.
The addition of spell rules, effects, and mechanics to an already complex game system might seem daunting, especially for newer players, but there are endless possibilities for ways to play a spellcaster, and it’s definitely worth braving a little extra reading!