1.Download The zip in the official website
2.unzip the zip and you get the iso file
3.insert the tf card to your mac
4.use disk tool to format the tf card to fat
5.open the terminal do the next steps:

• df -lh to see the tf card disk info

Filesystem     Size   Used  Avail Capacity   iused    ifree %iused  Mounted on
/dev/disk0s2  465Gi  393Gi   72Gi    85% 103167366 18760835   85%   /
/dev/disk2s2  *  *
 
   *    * * *   *   /

• diskutil umount /dev/disk2s2
• sudo dd bs=1m if=./2017-07-05-raspbian-jessie.img of=/dev/rdisk2

It takes a long time and you can get a cup of tea.

6.cd Boot:

touch ssh

7.Ready ! insert the card to raspberry and turn on.

The default username: pi password: raspberry use this to add root

sudo passwd root