It's mostly upvotes innit. Which is mostly comes down to comments of value and relevance, bonus if it comes with wit and (sometimes) humour about them.
Early on, it’s easier to comment than post so just jump into active threads and give useful, relevant replies especially if you have substantial knowledge about the topic. Timing matters too since I believe commenting on newer posts gives you more visibility.
As for not getting flagged or banned, it sometimes depends on each subreddit. For most parts, just follow the rules and tone of each subreddit and you'll be fine (more or less).
