You can access your user profile by clicking on your profile image in the right upper corner.
To add Skills, go to Skills tab on top of the page:
And select +Add skill:
You will be able to choose all skills and filter them by name, to add them, simply click +Add icon next to the chosen skill and finalise adding.
Those buttons next to the added skill will allow you to rate how strongly you feel about the topic.
Request a skill rating from your manager, Self-rate or Target (set a target of how good you want to become in the subject).