I normally start with:
1. Objective of social media marketing for the organisation (measurable).
2. Platform mix to achieve this objective based on usage insights like MAU or time spent on platform/s in your specific target audience.
3. Platform content consumption insights, general and specific to sector.
4. proposed content plan based on platform and consumer insights and how this will help meet the overall social media objectives (e.g. 20:30:50 ratio of content that promotes:informs:engages)
5. paid media strategy to make the most of content created targeted at achieving objectives with cost efficiency
6. KPIs and benchmarks
Hope this helps!