First, check if you are eligible. You can find that info here: https://www.gov.uk/employment-support-allowance/eligibility

If you are eligible, you can call jobcentre plus to make a claim: Telephone: 0800 169 0350.

It may be that you can claim for Universal Credit instead, in which case, check you’re eligible here: https://www.gov.uk/universal-credit/eligibility
And apply online here: https://www.gov.uk/apply-universal-credit