There is useful advice and self help online for all ages: https://www.nhs.uk/every-mind-matters/ and for children: https://www.oxfordhealth.nhs.uk/camhs/.
If you would like some professional support then look here for NHS provision of a number of services that you can self-refer to, including counselling, small groups and courses, and online therapy https://www.oxfordhealth.nhs.uk/talkingspaceplus/