I need a big oven if ever I want to put up a bakery. However, I do not have the money to buy an industrial or commercial oven. Electricity rates are also high in where I come from. So maybe, I can construct my own firewood/ clay/ cement traditional oven. Anyone can help me how to do it, please do so?

I do admire your industriousness with which you will find a way for you to make a living! You are a person who is akin to my own heart!

The first thing to be considered here is, do you know if you had such a home-built oven that it would be okay for you to sell things that you would bake in it? Do you know if it would be okay to construct such a thing for commercial business purposes? I mean, what kind of rules would you have to follow with this where you live, please?

Yes, you will need to check with the local health inspector who will have to approve the machinary you use in your kitchen. Places that sell food to the public have pages and pages of laws, some places even require specific cleaning materials and for some items to be made of certain materials