Chat with your zoning people. Where I live, we don't need a permit for anything under 200 sqft (12x16 = 192 sqft). You'll fill up a 10x12' shed in no time. If you have a 200sqft max, you might go for the 12x16'.

Check locally for a company who provides formed powder-coated steel roofing to roofing contractors. There's one here that will take your dimensions and provide materials (steel sheets, ridgecap, edging, screws, etc) and instructions for doing it yourself. If two women can put one of their roofs on, you can. Again, talk to a ROOFING MATERIALS company, NOT a roofing company. IOW, shop where the roofing companies shop for materials.

When you add gutters, make sure they tip in the right direction. Allow for spillover with your rain barrels.

Most thieves go through the window or door. If you don't want to invest in a strong metal door, get a cheap one and then ask your roofing materials company (where you got the roof stuff) for a sheet of unformed (flat) metal in the same color as your shed roof, and face the outside of your cheap door with it.

If you aren't putting in windows for light, you'll need power run to it. No light at all will get old really fast.

Plumb, level, square.

Sue