I tried zazen for a few months: I like it and decided to start it again just this week. Here is straightfoward advice on what to do: https://www.youtube.com/watch?v=nsFlrdXVFgo

if you don't want to watch the long youtube video read the following then skip to 8:20 where he explains how to think/what to do with your mind:

• how to sit your body: cross legged or lotus - but lotus requires flexibility and isn't necessary. straight spine, back and neck. rest your hands to make a ring shape. face a wall and shut your eyes. rock side to side a little then stop straight.
• how to think: first few times - it's initially very difficult to let your mind free itself of thoughts/chatter so a way to practice this is counting down slowly from 10, restarting if you stray from counting onto thinking about something else.