Making a ghost.....

What kind of ghost? There are a lot of different ghosts.

Some require a 3D app. Others require filming, rotoscope, or keying plus particle effects, glows, blend modes, and transparency. If we knew what kind of ghost you were trying to create, we could point you in the right direction. 

Adobe Animate would be an excellent tool to create cartoonish ghosts you could animate. The ghosts in the movie required a team of effects artists and days of labor for each shot, so don't expect to create something like this in a few minutes:

Embed* a sample image or two, describe what you are trying to accomplish, and we can probably point you in the right direction. It would also help us help you if we knew your level of experience with filming for visual effects. Most VFX artists face the most significant problems from not properly preparing for blocking, lighting, exposure, and even shutter speed when shooting the shots you need for the composite. I understand you are just getting started with After Effects. You might consider spending some time in the Learn workspace figuring out the UI and then looking at a few of the tutorials in the User Guide.

* The "Drag & Drop here..." area is buggy and should not be used to share images. Please use the toolbar or drag your images to the reply field.

Use shapes to build a green blob gost.  (layers for eyes, mouth, etc)  Pre-compose all the ghost layers and then use the puppet tool to animate the pre-composed layer