Where do I go online to find out the cause of someone's death?:(

Normally you don't, you may find a link to a news paper clipping that was published. Have you contacted family members to see a copy of the death certificate.

Usually newspaper obits don't include the cause of death. That will appear on the death certificate which you would have to order and pay money for from the county in which the person died. Or, like FrChuck said, ask one of the family members if you know any well enough.

I agree with Fr. Chuck and Wondergirl. Sometimes there will also be an article in the newspaper about the death, if the death was unusual. For example, a young woman died here recently, after attending a party, and because of the age and her lack of any apparent illness, an autopsy was done and found there was PCP in her blood, and that caused her death. She was not known for taking any kind of drug, in fact, she was very outspoken about the use of drugs and worked with teens. So it looks like someone deliberating put that in her drink.

So something different like that, the cause of death would be in the newspaper. Otherwise, as stated previously, you would pay for the death certificate or if you know the family members, to contact them. You could hire a private investigator and let that person find out and tell you. Guess that is one option.