How to vectorize an image in Illustrator.

Image Trace converts raster images (JPEG, PNG, PSD, etc.) into vector artwork. You can use this feature to trace and convert existing images, like pencil sketches, into vectors. Choose from various tracing presets for quick, desired results.

• Open or place a raster image in your Illustrator document.
• With the placed image selected, do one of the following:
  
  Choose Object > Image Trace > Make to trace with default parameters.
  
  Click the Image Trace button in the Control or Properties panel, or select a preset from the Tracing Presets button.
  
  Choose Window > Image Trace or switch to the Tracing workspace to open the Image Trace panel. Then, you can either choose a default preset from the drop-down menu or specify the tracing options.
• Adjust the tracing results in the Image Trace panel (Window > Image Trace).
• To convert the tracing object to paths and to manually edit the vector artwork, choose Object > Image Trace > Expand.

For more information, refer to this Image Trace help page.