emmie You should receive a letter four months before you reach pension age - I know this as I got one 3 weeks ago. It will say
"This is your invitation to get your state pension"
It will have your unique invitation code, such as
EMM285CY7K
which you then put into the online form, along with your bank details etc
If you're not entitled to the full pension because you haven't paid in for the necessary years, you can get it topped up to the full amount by Pension Credit, as long as you don't have savings of more than £10,000 and no other income/investments/pprivate pension which would reduce the Pension Credit. There is a Pension Credit calculator which will tell you:
https://pensioncreditcalculator.dwp.gov.uk/pension-credit-calculator.php
Good luck!