How do I tell my mom that I need help?

If you look back to my latest question, you'll see I'm having issues...and I want to talk to my parents about it ASAP, look into some professional help. Except... I don't know HOW to tell them. Last time I had issues like this, my dad kind of went into denial and my mom thought I was faking it/doing it for attention. So... how do I approach this and let them know I'm completely serious? And when (time of day and such) would be the best time? Please and thank you.