Not only will you want to landscape around the outbuilding itself, but you'll want to consider how all of it will tie into the larger yard. When you say "garden" .... do you mean vegetable garden, or just landscape? If you mean vegetable garden, explain what you have in mind (square footage, location, shape.) Given that the existing walkway is a completely utilitarian addition, you'll need to weigh if you want to redo some of it. It won't be possible to do a normal "foundation" planting for the building, where the walk abuts it. In order to understand better what you have, you need to submit better pictures of the building and space around it. Line up with the camera centered with the building face. Be back far enough that at least 15' of ground space shows (in the viewfinder) in front of the building. (You'll probably be 20 or more feet from building.) If space at the right and left end of the building does not show, pivot the camera, aiming in those directions and capture with shots that slightly overlap the center shot. (But be sure NOT to move the camera locations for adjoining shots.) You would capture the scene for both faces of the building that matter, separately. (In other words, the camera location will change.)

What kind of soil do you have? What is the sunlight like where you want to plant (full sun, part sun, shade, dappled, morning sun/afternoon shade, etc.)? Also, I notice that slope in your yard. Does the wide area in front of the shed have good drainage or does it tend to stay wet for a while after a rain? From the image you provided, it looks like you like the aesthetics of an informal, cottage-style garden. Is this correct? If you are intent on planting this year, spend the rest of the summer planning and digging your beds, amending the soil, coming up with a list of possible plants and their locations, and that sort of thing. Then you can start the actual planting this fall and next spring (or just wait until next spring entirely). It is certainly possible to plant things in the middle of summer, but it's not the best time because the heat tends to stress transplanted plants and because you have to be really diligent about keeping up with watering needs.

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You said "from a backyard shed" but do you mean "to a backyard shed"? I believe you can run 12/3 wire from your house to the shed if it's rated for underground use, but I doubt it can be simply plugged in to a receptacle.