Signing up to participate in a webinar is usually done online - and is fairly straight-forward. First, you will need to find the sign-up link. If you know any other participants, you can ask them to send you the link to register. However, if you don't know anyone else participating, you can often find the registration link by visiting the website of the webinar's organizer. In the event that you don't know the organizer's website, you can usually find it from a quick Google search.
Once you find the registration link, just follow the on-screen instructions to sign up for the webinar. A lot of webinars cost money to participate in, so you will probably need a debit/credit card to pay when registering. After you sign up (and potentially pay in advance), you should look for a confirmation email to verify that you are enrolled in the webinar. If you have any further questions about your registration, contact the webinar's organizing party.