Whether you’re playing a character that you’ve run for years or are sitting down with a fresh character sheet for your first session, it’s important to know your character. There are two aspects to this: the rules and the roleplay. 

On the rules side, make sure you know how the rules of the game work as they pertain to your character. That doesn’t mean you have to memorize the entire Player’s Handbook, but that you should have the bonuses and damage for your attacks already written down on your character sheet and that you read through any spells you have before you cast them so you know how they work. If it’s your first time playing a character, make sure to look over their character sheet so you’re familiar with their abilities, and look up anything you’re not familiar with or ask the DM if you don’t know where that info can be found. If you’re playing a character you’ve run for a while, then make sure your character sheet is up to date and that you’ve familiarized yourself with any new spells or abilities you may have gained. Don’t expect the GM or other players to know your character’s abilities for you.

The second aspect is roleplay, and in this sense knowing your character means knowing who they are as a person. Try to think of your character not as a sprite or avatar in a video game, but as a person with thoughts and feelings (even if those thoughts are “Krunk SMASH!!!”). Look at your character’s personality traits, their ideals and flaws and bonds, as well as their background and other life experiences, this gives you an outline of who your character is. Then, you get to fill in the rest with how you play them and have them react during the game, maybe with an accent or a mannerism you decide the character has, and by keeping that outline in mind it will give you insight into how your character would act and feel. This makes your character become more three dimensional and real, making the game more fun and immersive for everyone.