So nice to have a private outdoor space in the city! I love the columns and if it is possible to stablilize and reuse them, I would do so since they add nice character. It looks like you have a few different styles of containers/planters, and in such a small space, I would be inclined to choose just one or two for consistency. Right now you have built in brick planters, a half whiskey barrel, and a square planter. Because you have a small space, I would aim in the long run to limit the variety in general - not too many different types of plants, or too many types of seating, etc.

I would begin by evaluating what you have there. What is the thing in the middle of the photo that looks like a dark fabric hood? What plants are already in the space? (You can take photos and get IDs on the Name that Plant Forum. Take both closeups of foliage and flowers and whole plant photos to include.) Are current planters and benches stable and in good shape or do they need to be removed or replaced for safety or stability? What is the "floor" of space and is it in good repair? Is the soil in good shape or do you need to get bags of compost to enrich the soil before doing any planting? How is the space accessed and is there more space behind the camera? You say it is partly shaded. How much sun is there, and where does it actually fall? What you can reasonably grow well will be greatly dependent on light levels, so spend a day now and every couple of weeks until late June evaluating how the light changes with the seasons and over the course of the day. Do you have a way to get materials into the space? For instance, does everything need to come through your home, and do you have a vehicle to get materials to the space, or do you rely on public transportation? Is there access to water and power out there?

Then decide what you want from the space and how you want to use it. Will it just be for yourself or immediate family? A space for entertaining friends? A place for outdoor cooking, ranging from toasting marshmallows to grilling? A quiet spot for reading and contemplation? Will you be doing the work all on your own, or hiring a designer or have a work day with friends or family? Do you have time to do maintenance such as trimming and deadheading or do you want something that is low maintenance? Are there animal pests you want to discourage or animal visitors such as birds you want to encourage? Will you want lighting available for night time use?

How you want to use it and with whom along with the amount of work you are willing or able to do will determine how to approach the project along with budget. Once current plants are IDed and you have evaluated the hardscape that is currently there, you will need to decide which materials meet your needs as is, which can be reused in another spot, and which need to be removed. Then you can start adding to make it a space that fits what you would like.

One thing I've done is spend a lot of time looking at garden spaces and imagining being in them to get a feel for what spaces I find appealing. In your case, look in books (often libraries have a good garden section) and on line for images of courtyard spaces, small yards, and city gardens. Often British garden spaces are small and walled, so you may find good inspiration from British gardening books or websites on how to arrange small walled spaces like yours.

Please come back as you work on this and update or as other questions arise.