Literally use the search feature on Google or YouTube and you'll find loads of tutorials.
I did...nothing really conclusive came about that search. Theres one video on Youtube but its in Tergulu ( indian dialect ! )
Are you new to After Effects? If you have never used AE or another compositing application before expect to spend s good solid week learning how to use the User Interface, the basic principals of animation, the principals of layering, masking, removing backgrounds and blend modes.
I would approach the problem in this way:
- Carefully plan the shot of the body lying on the floor so you can match the camera angle, position and any camera movement later when you film the "ghost" standing up and laving the body.
- Film the ghost on a green screen stage or against a black background using the exact same camera placement, angle, and camera movement. The most important part of this second shot is to make it easy to remove the background, match the general lighting direction, and create an image that you can easily manipulate to get the look you want. Lighting skills are critical to make the ghost look easy to pull off in post production.
- When you start the post process you first trim the original shot of the body to include only the frames that you will actually be using. You do the same with the shot of the ghost. Now you layer the shots in a timeline with the body on the bottom layer and the ghost on the top layer.
- The next step is to separate the ghost from the background. You can do this using keying, rotoscope, rotobrush, or any one of a couple of dozen techniques to create a procedural matte so you end up with only the ghost on on the ghost layer.
- The next step is to carefully line up the ghost with the body, adjust the timing and start manipulating the image of the ghost. You may want to experiment with opacity, blend modes, blurs, glows and distortions. It would not be unusual to end up with several copies of the ghost layer with different diffusion, distortion, blur and blend modes applied to each to create your ghost image.
- The last step is to render a digital intermediate of this effects shot using a visually lossless production format and then edit that shot into your film in a non linear editor. You do not use After Effects to edit.
This technique is would be exactly the same in any compositing application. It's all about layers, opacity, blend modes, blurs and positioning. If the ghost is simple and was shot on a green screen and you don't need anything but a glow and some opacity you can even do the composite directly in Premiere Pro.
The key to making this work without a ton of extra work like motion tracking, camera tracking, 3D modeling and other time consuming and potentially expensive techniques is to shoot it right in the first place. If you have already shot the footage then we need to see what you have got to point you to the most efficient solution.
If you search YouTube for tutorials make sure you pick trainers that know what they are doing. Many if not most of the tutorials you find on YouTube are presented by enthusiastic amateurs that don't really know what they are doing. They can easily lead you down dead end paths, present ineffective techniques, develop bad work habits and generally waste your time. If you intend to use 3rd party effects to class up your composite then be sure to check the manufacturers websites for their tutorials.
Check out some of the excellent tutorials by VideoCopilot's Andrew Kramer: