For your own safety and wellbeing it is very important to choose carefully. A reputable counsellor will be a member of a professional body such as BACP and their training will be accredited by such an organisation. If it is BACP, they should also be registered with them (the voluntary register is relatively new). They should be having supervision, and you have every right to check this with them. They should also be covered by professional civil liability [indemnity] insurance, and you can also check this with them. A reputable counsellor will be happy to answer these questions. The ethiaal guidelines of their organisation will insist that they work under supervision and with insurance.
Choose from a reputable website. The Counselling Directory is a good place to look for a therapist in your area. (You can also find my profile there). Therapists can only be listed there if they have proved they are appropriately qualified, registered, insured, supervised etc. Their profile will tell you more about what they do, and whether they specialise in any particular area you are looking for help with (e.g. relationships) and you can also find therapists who specifically work with children. Many therapists will also have their own website link included for further information.