Be present. Which, seems obvious on the most basic level, but like… if you’re super into it, not checking your phone, or if online, watching twitter, listening to music, etc. you’re going to have a better time, and the DM isn’t going to have to explain stuff multiple times, or drop more hints than they wanted to steer you towards something. (If that’s their goal.)
Get into it, but don’t hog time. You’re here to act, essentially. Don’t worry at ALL about “oh, my voice isn’t good.” or “I don’t want to come off as some mega nerd, so I’m gunna be quiet.” This stuff is a ton of fun, and not everybody is going to be some rockstar voice actor. (The one downside to Critical Role getting a ton of people into it has to have been the self-doubt about acting it gives some people who aren’t as secure.) Just have fun.
Try to play the character, don’t metagame the best result for future adventures, or “winning” by some learned definition. Being somebody else is the win state. Maybe that’ll mean getting into trouble, and dying, maybe it won’t. (You can also ask your DM/GM out of character if you should reel it in if you’re worried you’re going too far off track.)
Oh, that’s another good starting point. Talk to your GM beforehand. Ask them what they’re expecting/hoping for from you. That’ll generally give them a chance to tell you “Okay, this is a good aligned campaign, you don’t have to do a voice for your character, but it’s fun, helps with clarity of who is talking in or out of world, but if you’re uncomfortable that’s fine…” etc. - Also, you can ask a few questions, which they may not think of which can help you determine some potentially critically important things. “Hey, there’s some subject matter I’d kinda like to avoid, Is this going to be a problem?” “Hey, I’d like to play an evil character, is that okay, or is that going to mess with this campaign?” (Sometimes it just will, and it’s better to find a campaign that it won’t mess up, for everybody at the tables sake.) But… often time just… being sure you sit down with the DM/GM and your other party members before the game starts, possibly before character creation, and asking questions, can give you a HUGE leg up on understanding expectations, where everyone is coming from, etc.
Have fun, ultimately is what the game master will want you to be doing. My tip as a player, towards that end, is, abandon your comfort zone as much as you can make yourself if you’re usually shy/quiet/etc. Worry about being your character, that’s the win state, not money or experience acquired. (But again, don’t be there shouting, acting out every single thing you do and hogging the mic to to speak, there’s a lot of people at the table, and if you’re talking half the time and aren’t the GM, you’ll want to check in with others. (Or, if you see someone not engaging, you can sometimes ask them about what they’re doing, what skills they can bring to the situation to get them involved,etc.))