Where should I start?

Linkedin Learning is also a good resource:

https://www.linkedin.com/learning/

I think there is a free trial.

After Effects is not a video editing app. You edit videos in Premiere Pro. 

After Effects is a compositing, motion graphics, and animation app deigned to create shots that you cannot create with the effects, transitions, and controls in an NLE like Premiere Pro. Most compositions in After Effects are one shot, occasionally, you will create a short sequence, but you should never try and edit a movie in After Effects. Most of my comps are under 7 seconds because I work mostly on movies and the average shot duration on a movie is just under 7 seconds. My comps are usually rendered to a frame-based and lossless production format like ProRez, and the final film is edited in Premiere Pro.

That said, start to learn the UI and basic workflow by simply opening the Learn workspace. If you save any of the projects in the Learn workspace or from the Home Screen, please save them to a new location, so they don't get lost when After Effects is updated. 

The nest resource you should explore is the User Guide. It is available on the After Effects product page and. on the home screen of this forum. You should be careful about the tutorials you find with a simple web search. Asking questions with the Search Help field in the top right corner of AE is almost always the best place to start a search for answers to questions about effects or the UI.

When searching for After Effects tutorials, ensure you vet the presenters. The number of views is no indication of the quality of the explanations or the workflow. Most of the tutorials you find on YouTube are prepared by enthusiasts, many of the recipes they share are poorly explained, and the workflows can be questionable. I spend a lot of time trying to answer user questions when they are following poorly presented tutorials. 

When you want to have help with a specific task, post a description of the design idea and the workflow you are following, and share screenshots that show the modified properties of the layers you are having problems with. Make sure you drag images you share to the reply field or use the toolbar to embed them, so they are easy to see. The Drag and Drop area uploads files for sharing, but it is not very efficient for sharing screenshots.

One last suggestion. Don't mess with the Composition Presets or Rendering Presets until you have a good understanding of video formats, codecs, compression, and standard frame sizes. Video is picky, and fiddling with the knobs before you know exactly what you are doing will get you into trouble. 

Start by reading the basics in the AE online help and then take it from there:

https://helpx.adobe.com/after-effects/how-to/aftereffects-workflow-terminology.html

https://helpx.adobe.com/after-effects/user-guide.html

Mylenium