If an issue is relatively minor, you may want to raise it informally with your manager (or that manager’s manager, if the issue is about them), and see if it can be resolved that way.
For more significant matters, you should refer to your employer’s Grievance Procedure. This should describe how you can raise a formal grievance. You should give your employer details (and any evidence you have) about your complaint, and how you would like it to be resolved. Evidence might include, for example, names of any witnesses to an incident.
You should expect the business to investigate and hold a meeting with you (at which you should be given the right to have a companion). The business will need to write to you with a decision, with reasons given. You should also be given a right to appeal the decision, if you are unhappy with the outcome.
If you feel that the way you have been treated is so serious that it goes to the root of your contract with your employer, please refer to our question “What is constructive dismissal and who can claim this?”.
Do also watch our video: https://tinyurl.com/y7rckph3 . You may also want to refer to the Acas Code of Practice for Disciplinary & Grievance Procedures, which underpins in the law in this area.