Asp.net Core APIs, Where to start form

Before asking the question i want to appolgy if its irrelevent, actually i want to learn about API's and want to get good advice from the young ones who are already attachted with it, while talking about API's what should learn first, where to start from... which technique will be the best.

I'm working in C#, Asp.net, 2 or 3 times i have developed small web services but now want to learn good and valuable regarding API's, 

Someone told me to learn Asp.net Core API's. Please Direct me with a starting point with your goodness.

Thanks in advance.