DM your characters. Know what’s on your character sheet, what your spells and abilities do, and what items and resources you have available to you. Write down rules you often forget and have them handy.
Talk with your fellow players in between sessions. Share ideas. Plan expeditions. Talk strategy. Discuss rules. Share resources you made or found online.
Session time is a resource. Sending the DM shopping requests and other downtime activities outside of a session means you can spend more session time adventuring. If you’re requesting something not covered by the rules, offer suggestions or propositions you would find suitable, along with any sources (books, Dragonsfoot, etc.). Minimize the effort from the DM needed in order to respond.
Work with your fellow players to decide on character selections, equipment, magic items, and prepared spells for an expedition.
Consider assigning roles during a session. Some groups may like callers and other roles, where some groups may not. Notetakers, quartermasters, mappers, surprise/initiative-rollers, and rule-checkers are often welcome.
Cooperation is critical. Friendly competition can add to the group, too.
Avoid unnecessary encounters. Distractions abound, but the main source of XP is treasure.
Imagine your character’s perspective. What they see, hear, and so on. This will help you ask the right questions and investigate the right things.
Be observant. Ask questions. Ask for detail if you’re confused or unclear about anything.
Be bold. An overabundance of caution means more chances to be found by monsters or more time for monsters to react to you. Delaying action can mean lost initiative. Not to mention, slower play burns session time and can cause your fellow players to lose attentiveness.
The balance between boldness and caution is a skill you’ll hone through play.