Now that you have a box to grow your garden in, time for what is next.
Where do you put it:
We originally went with container gardens because the spot for the garden just happened to be where we had just taken out a tree. The ground was shot and covered in pine needles. Do you know how painful pine needles are when they accidentally lodge in the hands while you're working dirt? All I did to get things ready for the containers was lay some weed block down underneath the containers. It keeps weeds from coming up and the plants from going down in to soil that is not as nutrient rich. It also allows drainage. Which brings me to the next point. They need to be basically level. Not perfectly, but basically. Since all I had was a pile of needle ridden dirt, I just moved it around until it was basically right. If you look at my first three containers, you will note that some are higher than others. That was on purpose.
If you want to do this on a hill, consider creating terraced containers. They would need to be longer, with the narrow side going up the slope, mitigating the slope to some effect. If it is a steep enough hill, you may need to do actual terracing.
Avoid putting them in a low spot in your yard. As in, one where water pools when it rains. Too much water could cause root rot.
This year, I put the boxes on (very weedy) grass. To deal with the grass around them, so I don't have to mow, now that I have things in place I'm going to kill it off or cover it with weed block and mulch (as soon as I can afford mulch).
What to fill it with:
I use a mix of a third to a half peat moss, and the rest compost. My composting adventure has not gone well for a variety of reasons, so I buy mine from the Depot. I get the compost plus manure, Mushroom Compost, and if I'm feeling rich, Moonure. Composting manure does crazy thing to plants. My uncle's composting horse manure grew the largest tomatoes I had seen before the 7 foot tall monsters I had last year.
I get enough for a good six inches of mix in each box. (that works out to about 8 cubic feet) This year, I skimped a little on the peat moss and used half a bag (it expands, a lot), and five bags of compost for each box. The peat moss helps add air and moisture retention that you would not get with straight compost. Compost alone can also be almost too rich for some plants. Not including the extra compost I got to help amend the older boxes and where I did some plantings by the house, two boxes worth cost me 30 bucks. If I had a trailer and lived in a city where they sell their compost, it would have been a lot less.
Ignore the information on the packets, sort of. You have nutrient rich soil so the largest concern becomes spacing plants so they get sun and don't stunt each others growth. This is where I suppose if I bought either of the books on the subject I might have more specifics. Instead, I sort of wing it. I mix heights so that things get good sun. I plant in rows going East to West because that maximizes the sun for the different heights. I thin things, but not as much as the seed packets. Worst case, if something isn't working, I can always take out another plant or too. Much harder to put them back. The only other thing I pay attention to is nightshades. If you want tomatoes, peppers, or eggplant, make sure you have at least two boxes. Rotate these crops so that they are never in the same box two years in a row. If you can do three years, even better.
It is entirely possible that the seven foot tall tomatoes were a fluke. I am hoping they were not. If they were, I may just be full of it.