I am using Onida's microwave oven. There is no option to preheat on that. Please give me some advice how should I bake things without preheating the oven. It's a 20 litre grill 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.
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.