Tax credits if you leave or move to the UK https://www.gov.uk/tax-credits-if-moving-country-or-travelling
How tax credits and other benefits affect each other https://www.gov.uk/how-tax-credits-affect-other-benefits
Do you qualify for tax credits: quick questionnaire https://www.gov.uk/qualify-tax-credits-quick-questionnaire
Correct a mistake on your tax credits claim form https://www.gov.uk/correct-mistake-tax-credits-claim-form