The most important thing is to understand what the problem is. Getting an accurate diagnosis will make sure that you do not aggravate the problem or turn it into a long term issue. Ideally, consult your GP who is likely to refer you to a Physiotherapist or Specialist. You can get an ultrasound scan and may be directed to an MRI depending on the problem. Avoid impact exercises such as HIIT classes or running as this may aggravate it. Try using a bike with a slightly higher seat as this will keep the knee moving. Once you have consulted a clinician they will be able to direct you which exercises to do and to avoid. Just remember that exercises should not make it worse.