What are the rules for starting conversations?

If you don't know the person, and you and he are just sitting around, say at a bus station or a waiting room, then it's fine to put out a conversational "feeler" to sound out if he feels like having a chat. Just say something unimportant -- the weather is always a good standby -- and gauge from his reaction if he wants to start up a conversation. The signs of interest will be eye contact with you, more animation, perhaps even a turn towards you.

I think there are some commonsense things to bear in mind as to whether it's not okay to talk to someone. Don't try to start up conversations where your intentions might be held to be suspect. And you should bear in mind cultural differences. Some races are more open to engaging in conversation with strangers, others less so.

Spindrif clearly has little life experience. Ignore her.

If someone speaks to your first, you should always respond in kind. But for ENGAGING others, as a male, you should avoid: obviously religious women (muslims in hijabs, Jews in shtetls, etc.), hobos (lots of mental illness and hygiene issues), thuggish types (ANY race), children, or people absorbed in their phones or other electronics (may be busy with work).

Otherwise, it's not a bad thing to try to strike up conversation, but keep in mind that not everyone is interested in small talk and many people will be suspicious of your motivations (girls may think you're trying to mack or flirt; same w/dudes, actually).

It is never NOT OK to talk to someone, and no, you do not have to know them beforehand, just start talking about anything that comes to mind; TV shows, music, movies, current events, anything.

EDIT: Hey "kaviani99999" you may think you have life experience but there is one thing you clearly do NOT have and that is MANNERS. In future, confine yourself to giving answers, not trashing answers others have given before you. You are NOT all that, my dear, and I have far more life experience than you will ever have. But thank you for your cogent comments which demonstrate you have more hubris than brains.