This is too massive a topic to answer here, but can at least provide a few tips and point you towards relevant resources.
You could try approaching a social media marketing plan using the SOSTAC strategy framework (here's an infographic you might find useful: http://www.smartinsights.com/managing-digital-mark...
1. Situation Analysis
I won't explain it fully here (check out that link and do a Google search), but approaching any marketing plan in this manner helps provide a stable framework to work within.
As an example, for Situation Analysis, you could ask yourself:
Which networks are we on already?
What is our follower/fan base on those networks?
How often are we posting or contributing?
What are our current engagement metrics?
What traffic does social currently bring our website?
You can work from that starting point of establishing: Where are we now?
Image credits: http://www.stateofsearch.com/seo-within-the-sostac...