The hardest step towards getting help is recognising something's wrong. If you can't help your loved one recognise there's a problem, then encourage them to see a professional who can assess symptoms and provide a diagnosis and treatment options.
A general practitioner (GP or doctor) is the best person to see for non-urgent cases. GPs can assess whether symptoms aren't caused by any other illness. They may then refer to a mental health practitioner trained in treating mental health conditions; or if they know a lot about mental illness, they can provide treatment themselves.
There are numerous professionals who can help, including:
These professionals are trained to help people with a mental illness and will offer treatment to help them recover or cope better. Typically they'll start by assessing the problems, then offer different types of treatment like 'talking' therapy (eg. counselling or psychotherapy) or medication. They'll often offer both as it's been found to be a more effective means of getting better.
If your loved one is particularly unwell, they may need more intensive treatment or care in hospital.