I think 80% of the folks here make their own compost. It's free and it works well so it may be worth looking into it. The only advice I am giving right now is to plan so to have the compost heap on the bare soil to improve drainage and to allow earthworms and other beneficial creatures to pass into the compost once it has cooled down.
Lacking compost, the second best (but expensive) thing is manure. There are many kinds to choose from but among those available commercially horse manure is the best followed by cattle manure. A 50/50 mixture worked into the ground in late Fall/early Winter (just like compost) will do wonders. Sheep manure, provided you can find any, is better and can be used alone and rabbit manure is the absolute best but it's only available in very small amounts and it's generally hugely expensive unless you eat a lot of connies.
As per containers perhaps other folks have different experiences but my advice is to use a good potting medium the first year followed by a 50/50 mixture of old potting medium and compost the next. However remember that containers do not allow any plant bar the puniest to properly develop root systems and to make things worse more frequent waterings will wash away nutrients: that's why you need to provide a liquid fertilizer regularly during growing season. Vegetable-based ones are available and seem to work pretty well.
Finally one note: never rule out synthetic fertilizers. If used properly they will literally multiply your yield, especially on a tiny plot. A 10-10-10 or 12-12-16 high quality fertilizer will literally do wonders once you learn how to use it.