You can't go wrong with compost. I started with the stuff available for free at my town recycling center where people bring their yard waste. It gets turned with an earth mover every week or so. The only issues with it are rocks, pine cones, chunks of wood and other assorted crap which is a PITA to screen. I ended up using a little screened as a base and started composting my own leaves and grass clippings as well as kitchen scraps (no apple cores or corncobs or any corn as that attracts deer) and my used FFOF. Didn't take long before I had that black gold with a mini worm farm inside

. I turn it once a week and each time I see lots of redworms and a few big nightcrawlers. If only I could find a way to get just their poop out for castings. I mix it with vermiculite, about 70/30 compost/verm. and about 60/40 FFOF/compost mix. You can find tons of examples of DIY compost bins online.