If you like to have an expansive garden, plant some of your favorite shrubs around the perimeter of the tin shed. Use shrubs you know will grow large enough to conceal the shed entirely. Leave a little space between them, though, so you don't have to struggle to access the shed. If you do not intend to access the shed, feel free to put the shrubs as close together as their space requirements allow.
Setting up a trellis around the shed can create a pleasing visual effect and give you a place to start growing some impressive vine plants. Place the trellises right against the sides of the shed if your concern is the exterior look of the shed alone. If you want the shed concealed entirely, set up trellises around the perimeter of the shed that stand taller than the highest point of the shed. Plant vines like ivy around the base of the trellis to obscure the shed. Avoid thin vine plants that do not hide the shed well.
A greenhouse is a useful structure for many plant lovers. A large enough greenhouse can help disguise your old metal shed as well. Construct the greenhouse over the shed to obscure it from view completely. As a bonus, you can use the shed for storage of garden tools inside the greenhouse, so you have your tools readily available. The humidity in the greenhouse may cause the metal to rust at an accelerated rate, though, so examine the shed for any deteriorating sections that may lead to structure damage.
False Privacy Fence
If your shed sits near the far corner of your garden, a fence may be the perfect solution for hiding it. Build a privacy fence that separates the shed from the rest of the yard, concealing it between the new and the old fence. Leave a gate in the new fence so you can go back and access the shed as necessary. The location of the shed is easy to discern if the shine of the metal shows through the fence, however. Paint the sides of the metal fence as well as the roof to keep it from reflecting light.