1) Normally you will need to go and see your GP. They can then refer you to specialists if they think necessary (usually a psychiatrist) for official diagnosis and to discuss therapy/treatment options. I have never heard of diagnosis online and would be concerned that they could not help you as well as a real life doctor could.

2) It would only be you and anyone you choose to tell normally. If you are under 18 your parents may need to consent to some medications etc but you can discuss this with your doctor. Who you tell outside of that is up to you.

3) No need for them to know, unless you tell them. Some professions, especially if you are working in the field of mental health or counselling prefer if you do tell them but usually only so they understand and can make adjustments to ensure you don't become overwhelmed. And you wouldn't generally need to do that until they offer you the position. They cannot discard your application just because of mental illness.

Hope this helps