supervised land over classification

I am doing a project classifying land cover types and I was wondering if/how you could do supervised classification. Supervised classification being the manual selection of pure pixels that are within a specific class, and tensor flow uses those values to identify the entirety of that class within a whole image.

An example would be selecting 20 groups of pixels that are all trees, 20 groups that are all grassland, and 20 pixels that are all water, and then the entire image is categorized into one of those three classes.

Thanks for the help!