all projects save on desktop - how to disable?

Any time your starting a new After Effects project, take the time to create an organized folder structure first.

For example, let's say you're going to aniamte some images of flowers in After Effects that are saved as PNG files.  Create a folder named"Flowers".  Inside that folder, create the following three folders: "AEPs", "Source Fotoage", and "Renders".  Move all of your PNG files into the Source Footage folder before you import them into your After Effects project.  If you add more images later, make sure that you move them to the Source Footage folder prior to importing them.  Save your After Effects project (the .aep) to the AEPs folder.  When you're readyn to render a Comp, set the "Renders" folder via the "Output to" option in the Render Qeuue panel (or set it as the destination if using Adobe Media Encoder).

Repeat this for the next project.  And the next.  And the next.

Working neatly in AE is 100% up to us users.  

When you select Save you can choose any folder. It is no different than any other application. The only reason your projects are going to the desktop is that you have not carefully looked at the options and chosen a different location.

Please, open the Learn workspace and get acquainted with the User Interface. It will take about 15 minutes. Then spend some time with the User Guide. You can't learn how AE works by just poking around and following a few YouTube tutorials because, frankly, most of the tutorials that turn up in a web search are prepared and presented by enthusiasts who stumbled upon some recipe somewhere and most of the workflows are less than ideal and most of the explanations are terrible.