#1
Writing a post that says you're a newbie is not obligatory; if you stick around long enough to write more then three posts most of your readers will never see it any way. Using the first paragraph or two of you first post to introduce yourself is annoying, since this means the trimmed version of your post (what is generally seen first) will only include your self-introduction and won't say anything about the body of your post. If you must introduce yourself and/or explain your background or position, you should write a profile, but that is optional, since people will quickly figure out your views by reading your posts and the comments you leave.
#2
This is not myspace. Anything you write on here should be progressive, which means it needs to have at least a semi-serious point. This doesn't mean it needs to be about politics, or even relate to a news story; it just needs to be something important to you that does/can/will impact the world beyond yourself and your friends.
#3
Just because someone else wrote a post about a particular topic and it got a lot of reads does not mean you should write a post about that topic. You should choose topics that are important to you, especially topics you are knowledgeable about and/or not a lot of people are familiar with.
#4
Make sure your comments are in the right place. Clicking the “reply” button immediately under a comment will indent your comment underneath it; this button should only be used if your comment is in reply to the comment above the button. Clicking the “new comment” button or typing in the comment box at the bottom of the page will not indent your comment or attach it to the comment(s) above it; don't use this button if your comment is in reply to another comment, since that makes it harder to keep track of what everyone is saying.
#5
This isn't an instant messenger. Your post and comments should be in sentence form, not use acronyms or internet slang, and be proofread for spelling and grammar. Using short to medium length paragraphs also makes it much easier for people to read longer blogs. Rachel Setzer's post http://www.progressiveu.org/153651-blog-etiquette-or-how-to-write-something-people-will-read [0] talks more about proper grammar, and why it is important.
#6
Expect people to disagree with you. It doesn't matter what you say there will be people who think it's wrong, but try to be as respectful as possible while disagreeing. Name calling and making the argument personal are inappropriate.
I hope you find these tips helpful, and enjoy blogging for progress.