How can I make cakes in 20Ltr ONIDA Black Diamond conventional microwave oven?
I don't have specific instructions for that oven - and it is likely you won't find any recipes specific for that oven unless it came with a cook-book when supplied. However, it is possible to cook cakes in a microwave oven, there are many recipes available on the internet (e.g. this one).

You will need a recipe for a cake that is designed to be cooked in the microwave; conventional recipes do not usually work without some adapting.

Microwaves generally don't come with a temperature setting because they don't work off transferred heat (infra-red radiation) like a convection or fan oven, instead they use microwave frequency radiation, which gets absorbed by the items in the microwave cooking them directly - water is a strong absorber of microwaves, meaning it gets heated rapidly compared to a transferred heat system. Microwaves do however come with a power setting - again, this is a little confusing, because what it actually is, is a timer that switches the microwave generator on and off for different periods of time - longer breaks between it being on at "lower power" settings. These power settings are often useful for baking, especially if you are trying to cook a large cake.

You will also not be able to use your metal or teflon-lined metal baking pans in a microwave. I recommend using a well-greased glass or ceramic bowl or a silicone baking mould.