First make sure you have built ycmd; see the top-level README for details.
Then install all the Python requirements with pip:
pip install -r requirements.txt --use-mirrors
Then just run ./example_client.py from the console. It will start ycmd, send
it some example requests while logging the full HTTP request & response and then
shut everything down.
The best way to learn how to use ycmd is to play around with the example client; tweak the code, send other requests etc.
NOTE: Everything in this folder and below is licensed under the Apache2 license, not the GPLv3 like the rest of ycmd.