Social Media Marketing

I’m starting with my company. And I’m about to establish a profitable social media strategy. So I have a few questions to ask:

How do I choose the right platforms to work on?
What are the ingredients of a good social media strategy?

To make an informed decision, I visited a site: (link removed by moderator). I found some pretty clear information and advice, but there’s still something that remains unclear to me. So I think a topic like this one on a forum could help me understand it completely.