Re: howto find a htc desire 820
Keep looking on
Android Device Manager. When it's switched on it'll show its location (unless the thief removed the SIM card, turned it on, put it into airplane mode, then set it up for a different account and phone number, in which case it's gone).
In any event, report it to your local police and report it to your carrier. The police report will or won't be helpful to one degree or another, depending on where you live, but reporting it to your carrier makes the phone useless. It'll be put on the blacklist, and it won't connect to any network.