Hi - Mental health is an intangible problem; it's not measurable and so it's very hard to identify. Many of the symptoms of mental illness are subjective, meaning that the sufferer is the only one who can experience the problem. There is no physical evidence that cannot be explained by some other possible cause, such as tiredness, sadness, lack of interest. Some people are tired because of unidentified physical reasons; some people are sad because of stuff going on in their lives, and some people are just bored and uninspired.
A frequent problem occurs when someone getting help through medication feels better and stops his or her meds thinking he/she is cured.
As for getting help, some health insurance pays for mental health services, and some doesn't. However, there are Social Service agencies that are publicly funded where free or reduced cost mental health assistance is available. If you're in school, your counselor could probably provide some referrals or you could look online at your city's or county's Social Services offerings.
A good way to start is with your primary physician, who can do a physical exam to rule out any medical issues and who might be able to arrange a mental health practitioner's visit.
If it's you needing the help, then good luck and don't give up. If you're asking for a friend, then you're a good friend for trying to help.