Gardening on a budget question?

Dh and Db built me a raised garden box from scrap wood (I have been asking for one)! We put it in the corner of our yard, where it will receive the most of the Eastern sunrise (and be most out of the way).

Its a modest-sized planter approx 2.5 feet wide and maybe 8-9 feet long. It stands about 2.5 feet tall.

I am trying to work out two things right now: Inexpensive ways to make cheap dirt into gardening soil by spring, and which veggies or fruits to plant which will give me the most bang for my buck.

I want to plant items that will be an inexpensive start, but yield us a fair amount of produce next year. Its a hobby garden or sure, but I don’t want it to cost more to grow our own veggies than buy them from the store.

What tips do you all have? Has anyone tried to start their gardens from seed, which I gather is the least expensive start?

I am in Houston, I guess we are zone 9-ish.