There are a variety of ways to track the key metrics of your marketing effort but these all depend on what sort of business model you are running. As it’s not very clear which model you are running (subscription model, e-commerce model or 2-sided marketplace), I can only provide some general advice.
Firstly you will want to track metrics around your product-market fit. If customers are not coming back to buy more courses from your site, then its important to find out if customers are satisfied with your courses, tutors or pricing. You should be tracking
Satisfaction scores of your content, tutors and pricing. Sending a simple survey after customers complete a course will be a great way to get satisfaction scores on your content, tutors and pricing
Net Promoter Score (NPS). Tracking NPS can be a great proxy indicator for product-market fit. Customers are only going to recommend a product with which they are satisfied. Sending a regular survey to all revenue customers will be a good way to track NPS.
Sean Ellis Product Market Fit Question. Sean Ellis of http://growthhackers.com advocates that you ask users of your product this question. "How would you feel if you could no longer use [your product]?" The four possible responses are "Very disappointed", "Somewhat disappointed", "Not disappointed (it isn't really that useful)", "N/A - I no longer user [your product]". Products for which 40 percent or more of users reply 'Very disappointed" tend to have product-market fit
Secondly, you will want to track the effectiveness of your campaigns for getting existing customers to come back and buy again. Good metrics to track around this area are
Return Rate of Visitors to your site - Returning Visitors (current time period) / Total Visitors (previous time period). Google Analytics is the tool of choice here
Clickthrough Rates (CTR) - Tracking CTR of your email campaigns to existing users is a good indicator of how effective your campaigns are. If customers are not seeing or clicking on your campaign emails, then it will be difficult to get customers back to your site.
Finally, you will want to track metrics that help measure actual retention. Some key metrics here are
90-Day Repurchase Rate - What percentage of your customer base make another purchase in a 90-day period.
Retention Rate - What percentage of your customers are actively using your product. The definition of Customer Activity can vary depending on your model. You will have to decide within your team what an Active Customer is.
To conclude, while there are a variety of metrics you can track on your dashboard, it is usually advised to focus on improving the One Metric That Matters Most (MTMM) at any given time. The MTMM is the metric that can give the most return on investment for your business now. Some excellent books to check out to learn more about metrics are the Lean Product Playbook (Dan Olsen) and Lean Analytics (Alistair Croll & Benjamin Yoskovitz).
Hope this helps.