Start by checking to see if you are eligible here: https://www.gov.uk/universal-credit/eligibility
If so, complete the online application here: https://www.gov.uk/apply-universal-credit
The system is currently dealing with a heavy backlog, so get the claim in as soon as possible.
One quick tip: The DWP have said the UC application website is least busy after 9pm (with the absolute quietest time at 4am). If possible, complete your claim after 9pm for the smoothest process.
The digital skills people at LearnMyWay have lots of information about UC and how to manage a claim in the form of a short course here: https://www.learnmyway.com/courses/universal-credit-a-how-to-guide/
And, of course, Quids in! has a guide to the whole system available in its Store here