Today i buy new apple watch buy screen show red snake how can i do please tell me

You will need to put it in the charger for a long time

If your watch requires charging, a red lightning bolt  appears on the screen. (You might need to click the side button to see the watch face.) When your watch is charging, the red lightning bolt  changes to a green lightning bolt . If your screen is blank or you see the charging cable icon and a red lightning bolt  on the screen, charge your watch for up to 30 minutes. 

That is power reserve mode;

To turn off power reserve mode,press and hold the side button for about 10 seconds, until you see the watch restart and the Apple logo appear.

Keep in mind that if your battery is almost completely empty, you might need to charge the watch before it will restart normally.