How do I set a low but steady speed for taxiing?
At the moment I race down the taxiways as if I am going to take off and have to brake like hell at the end of taxiway.
just keep the throttle low!!! or on civil jets set the auto throttle
How do I set autothrottle coz I turn it on on the autopilot panel and it does nothing.
what a/c are u flying
On taxi FAA mandates that you should never exceed 25 kts Ground Speed (GS as indicated in the GPS of default aircraft). Keep the airplane at minimum power, and you'll get rolling just fine.
For the auto-throttle, that's there to hold the speeds you set into the autopilot. If you flip it on, you can then engage things like the "TO/GO" buttons (to get full automated take off power), the "SPD" or the "Mach" buttons on your autopilot to have it hold the speed you program in there.
You can't use it to taxi though! Just to make your flying more convinient once in the air.