I'm making this post because I keep seeing a lot of spam comments by newbies, thinking they'll be successful on Steemit by commenting spam and telling people to follow them.
Not only that, I find it amusing to see people commenting "great post" 30 seconds after it was published, I mean, no way you already read it, so why would you comment that? You need to understand that we see things like that, and not only we won't follow or upvote you, but we might flag you.
Why might we flag you?
We might flag you because after working on a post, wanting to get valuable comments depending on the post's goal, and then seeing a type of comments like " great post follow me please " " follow me I follow you " is very, very annoying.
Understand the community, no one wants to see these types of comments on his post nor on Steemit in general.
If you see spam, please report it to 
What are spam comments?
Here are some examples of pure spam, please don't comment like this, and if you did until now, stop doing so, trust me, it won't help you
Be aware that if the post is educational, and you comment "very educational post, thank you" or something like that, it will be considered as spam to most of us, even if you read the post. because we can't know you actually read it, when we see comments, we want to see valuable comments.
What are valuable comments?
Valuable comments are comments that add something to the post, it can be giving your own perspective, criticising, recommending on stuff that is related to the post, asking a question that is related to the post and so on.
Here's an example of a good comment, he/she share his/her own experience and engage with the author, plus got an upvote.
Stick to this, don't comment spam, and I assure you that you will stop getting flagged and you'll get upvotes instead, not only that, you will get new followers, sometimes even followers with a nice amount of SP that can help you make a progress on Steemit.
How can I find where to comment?
You want to only comment on things you can add value to, which means, you need to gather the things you're good at or have knowledge at and go to to the specific tags of these subjects.
When you find the perfect tags for you to be active on, go to the "Hot" and "Trending" sections of them, and start reading the posts and then start making valuable comments.