Greetings! I'm an RPer though for different fandoms outside of MR and would like to try my hand at RPing in the Streets forum.

Problem is that I don't  know how to start or how to respond to established threads.  How does one go about it and what can you do there without looking like you interrupted something or posted randomly?

If you'd like to start a thread in the streets, I'd suggest doing anything you are interested in (while staying within the 1920s/30s era *roughly*). Reading other threads first can be helpful to get the gist of things, and can serve as an inspiration for ideas. 

In the case of responding to threads, the easiest way to get in on a thread without offending anyone is mailing them and asking if you can join. From there, you can communicate as to what happens next, and so on. 

You certainly can randomly join other people's threads, but I would definitely suggest reading the full thing beforehand to understand the context and situation. It is within reason to RP without RPing other people's actions, instead more like a call and response in the sense of providing a new development that they must react to (again, within reason). 

TLDR; communication helps, also ask (or make) a friend to have some fun RPing and see what comes of it!

Thank you for the sound advice! Hopefully I could be able to make a good rp friend if ever I do decide to embark RPing in the Streets