Preheating Onida convection microwave oven
I assume you are using one of Onida's "convection" microwave oven.

Here is a link that describe how to pre-heat your oven.

http://foodomania.com/setting-oven-temperature/

In summary: Select the "bake" option. Hit Enter/start once, then choose the temperature. Hit it once again, which will let you select the time. For preheating, leave it at 0:00. Hit start/enter again. When your oven beeps, it has reached the temperature. Hit Stop and bake.