As general points, cooperate with investigations and proper processes that are followed, maintain confidentiality and tell the truth.
Gather evidence that you may have that explains your (alleged or actual) misconduct or performance. Keep diary notes while any events are fresh in your mind.
You may want to refer to the ACAS Code of Practice for Disciplinary & Grievance Procedures, which underpins in the law in this area. This is what your employer will be expected to follow.
It may be that you are unhappy about the process being followed. For example, if you believe the person leading the process will not give you a fair hearing. It may be appropriate for you to raise a grievance about this, and for your employer to consider this before proceeding further.
Consider appealing against a decision made against you. This is a key right in the ACAS Code.
In some cases, particularly if you believe the allegations are incorrect or you believe there has already been a decision to ‘force you out’, it may be appropriate to submit a Data Subject Access Request to your employer, in order to obtain disclosure of information they hold about you.
Particularly if you are subject to a formal conduct or performance process where you have been informed you may be dismissed, we recommend you take advice from our specialist employment team in advance.
You may also want watch our videos: https://tinyurl.com/ybcycwqh and https://tinyurl.com/yc5auuyn