Roleplaying is the heart of the game, and it is what truly separates it from a video game. Stepping into character and making choices as someone else is a core player skill. Like any skill, it can be hard or awkward at first, but it can be learned.
A good backstory can help with roleplaying, but it is not a replacement for it. Your character's past informs their opinions and beliefs, but does not dictate their behavior.
Just like how the DM prepares sessions, players should prepare by thinking about their character. Fifteen minutes before each session goes a long way. Consider their opinion about events that have occurred, other PCs and NPCs, and the party's current mission. Perhaps you can also think about a scene you would like to play out with a party member. Don't hesitate to write these down - I promise the DM is also referring to their notes during the session.
What Roleplaying Is
Making choices
The most important part of roleplaying is to make the ingame choices as your character. This is hardest when the character would make a bad choice - which is where the best moments often are.
Don't be afraid of having your character hold beliefs you disagree with or know are erroneous. This is especially true in the beginning of a campaign, they can always reconsider their beliefs as part of their arc and growth.
Considering the world
While it isn't possible to know every piece of lore about the world, thinking about how your character fits in the world is the first step to making choices. You should probably have read (and considered how your character feels) about your class, heritage and culture, home town, deity, and faction. Ask the DM for relevant links if you are unsure what to read about.
This is also where joke characters might be weeded out. Otto the Fart Wizard might be very funny in a one-shot set at a circus, but probably doesn't fit for a longer campaign.
What Roleplaying Isn't
Acting
Good roleplaying does not come from Oscar-level performance. It is okay to describe your actions in third person, or to summarise what you say to an NPC. Especially when playing online, it can be harder to interpret a performance.
Speaking in a voice
While a good character voice can elevate your character, it is far from a requirement. Do not be afraid to make and play characters of the opposite gender or a monstrous race just because you don't sound like that. A funny voice is also not a replacement for actual character traits.
Things to focus on
These are some concrete tips for what always works well and builds party spirit.
Describe your actions
Embellish the descriptions of what you do, both in and out of combat. Do not view your character sheet purely as a list of buttons to push, especially out of combat. In combat, things are a bit stricter, but you should still think about ways of being creative. As long as it is your turn in combat, you have a lot of leeway in how you describe your successes and failures, as long as they do not incur mechanical benefits. This is your opportunity to describe exactly how cool your monk looks when they roundhouse kick the orc off a cliff.
Instead of "I attack", try "I shout the warcry of my order, before closing the distance and swinging with my greatsword: Deus Vult!". "I roll Persuasion" is not as cool as "I compliment the mayor on how beautiful his town is, before pivoting to my research and how access to his archives would be beneficial to both of us." Describe your character looking, smelling, and listening, don't just roll Perception and ask what you notice. A good description of your actions might give you advantage or even bypass the roll entirely.
If the DM calls for a check you did not foresee, you can always clarify what you intended.
The only pitfall here is to take too much time with your descriptions. Save your longer ones for when it truly matters. A quick sentence goes a long way, and you can prepare a few general descriptions of your common maneuvers before the session.
Interact with the other players
A common beginner oversight is focusing too much on interacting with NPCs, and not enough with your fellow party members. Instead of asking a question to the DM, try asking it to your party. The more you speak to the other player characters, the easier it will get.
React to what happens
When something happens that your character has a strong opinion on, make sure you express it! You don't know how many chances you will get to express your feelings on a particular subject.
However, be wary of overinserting yourself. If someone at the table is having a character moment, not every other character needs to be a part of it. Sit back and enjoy the show, and be ready to pick up the next scene when the current one is over.
People are complicated
Do not be afraid of contradictions. People do not always behave according to logic, and your character can too. Let your character be formed by the adventure - by breaking with your character's past, you can achieve growth.
This is perhaps most exemplified by alignment. It is important to understand that alignment is descriptive, not prescriptive. If your actions change, so does your alignment. Your alignment never bars you from taking a certain action (except for certain magical phenomena).