You may have a back yard or a patio, but you want a garden. no problem. Use containers. But there are some simple rules for a container garden.
The first is that containers should be large enough to take the plants that you want to grow in them, and you must leave enough room for growth. You will have to "pot on" certain plants as they grow. When you judge that they are becoming too big for the plant pot, gently take them out, and pot them in something larger. When you do this, fluff the roots out a little, because otherwise they tend to cirlce round the pot and trap themselves.
Always leave a bit of drainage space at the bottom of the pot, because in a severe rainstorm pots can become flooded, drowning the roots. That is why it is always a good idea to have containers with some holes in the bottom to allow water to escape. You never need to over-water any plan. Some containers for indoor use have no drainage holes, so do not use them out doors.Some crocks [broken pots] or shards of brick placed at the bottom will allow drainage space and prevent the roots from becoming flooded.
Ensure that the container is placed in sunlight according to the needs of the plant in it. Not all plants enjoy full sun, so research the plant that you are potting and decide where to out it.In addition to this ensure that you place tender and half hardy plants out only after the last frost date in your area. Be aware, in Britain gardening manuals seem to be based on the south, so their dates are for that reason. We in the north have to wait a little and use out judgment.
The big problem with containers is that they dry out easily, as the plants have no access to soil water, so you need to ensure that the soil does not dry out.
In addition you will need to ensure that the nutritional content of the soil is refreshed annually. A little ferrtiliser every few weeks will work nicely, but it is a good idea to refresh the soil yearly, potting on the plant.
Veegtables can be growin containers as well as flowers. Potatoes grow in sacks or converted bins. On my allotment I use one ton rubble sacks. The reason for this is that one end of my plot is alongside a road which is lined with large trees, which drain the soil and make that bit useless for growing. I have my compost heaps on part of this space and grow potatoes in containers in part of the rest of it. I can get good early potatoes ahead of the rest of the allotment, as the soil in the sacks warms up quicker than the ground soil