We are all lucky to have you in this platform. Your experience is evident in this intro post. It's just a matter of time and you will do really well here.

Here are some tips:

1. If you have the time, it also looked like you like reading; spend a couple of hours to read the steem whitepaper to read about the technical side of steem.
2. Invest when you get comfortable with the platform. This can be done both via using money or really taking time to post regularly. When you hold a larger steem power, your influence in the platform also increases.
3. Follow likeminded people, a community in close proximity to you, and try to bring in people close to you into the platform. This will make your journey more enjoyable.

About your questions.

1. It is okay to post your past works here. You will have to site the source if that can be found on-line, the community frowns at plagiarism. It will be really good if you can add some new updates if the article is old and newer information are available.
2. It will be advisable to post mainly on a specific niche, then alternate with other things. You have to be known for something here that your followers will look forward to. Stay within the intersection of these three, your passion, what you are great at, and something you think the community will appreciate (less important than the first 2). For me just bring the awesome you!
3. Who to follow really depends on you, make it a mix of people that inspires you, people you think have great potentil, and as I said people with like minds.

English is not my first language so I apologize for some errors.

Most importantly steem on!