If you are familiar with Google Analytics, the best place to start is with the basic attribution reporting section. Look at which channels are better at assisting conversions and which tend to be better at closing conversions.

Then, tie it up with whatever your current marketing goals are.

So if for example you see that social media is assisting a high number of conversions, but you've spent no marketing resource on it to date, you might want to review your social media activity to capitalise on this important touchpoint in your customer's conversion journey.