First, invest in small-scale businesses.
Small-scale businesses in Nigeria employ more people than large-scale businesses but that sector is neglected. The government should make programs that fund these businesses. An economy cannot be big if these small-scale businesses don't succeed, no matter how much the big corporations do.
When the cost of production is too high it takes the small businesses out of business and the few that survive increase the price of their products so they can cover the cost of production and make a profit. This cuts across all sectors not just agriculture.

I don't believe focusing on Agriculture alone would do much because the inflation in the country affects everything so even if the agricultural sector is well-funded, people still don't have money and prices would still be expensive.
One problem causes many other problems, and so does one solution.
The economy is intertwined so solving one problem won't mean much if other problems affecting it are still very present.