Credits rating are how lenders such as banks assess your ability to repay loans. This is really important as this is one of the major factors when getting a mortgage.
Credit rating is measured on a combination of ways, such as how much money you owe, finance deals, contracts and how much you are able to borrow on credit cards.
The main credit reference agencies are Experian, TransUnion and Equifax. They combine data from a bunch of sources such as electoral roll, court records, address and account data.
How to figure out your credit score; there are multiple companies that can give you a monthly credit score, each one has a slightly different way of scoring. Once you receive your score you will be able to see where to improve. Most credit rating companies will give you advice on ways to increase your score.

It is important to start building up your credit rating as early as possible, stay away from debt in any form. Getting a credit card is a good place; you just need to ensure you pay it off on time every month. Paying things monthly will also help, such as bills. Increasing your credit limit can help, just don’t spend more unless you have to.
little things will also help, such as making sure you are signed up to the electoral roll and putting stuff in your own name, such as a phone bill. The pros of good credit mean will you get accepted to more mortgages, have higher credit limits (borrow more) and have lower interest rates.
I would recommend experian to check your credit rating; https://www.experian.co.uk/