I've built two sheds almost exactly like you describe.

Just a couple of things to think about

What are you going to build it on? Concrete slab, dirt, grass, slope, etc? That's probably the first thing to think about. Most important thing is to keep everything level and square, it'll save you a lot of headaches later on. If you're building on flat concret, that's a lot easier than on a grassy slope cuz you won't have to make a level foundation.

Once you have the foundation set the actual construction is fairly easy. Total cost (about 6 years ago), came out to maybe $1200? +/-, I don't remember the exact cost but it was roughly half of what you would have paid at Home Depot for the same size, and much better quality.

Didn't really follow any plans, just estimated how much materials, then started building. 2x4's on 16" centers, 2x6 rafters, exterior 3/4" plywood panel walls w/1x3 molding around the exterior edges to give it a finished look. Roof was just plain asphalt shingles over plywood. I think it took about 2 days for actual construction with 6 people, then another couple weeks to finally get around to painting and finish roofing. Added a couple of wall vents and a roof ventilator to help with the temperatures in the summer time, it does get very hot inside. Doors were also 2x4 and plywood construction. Most importantly, while you're designing it keep in mind plywood comes in 48" widths, so plan the stud locations carefully so you don't need to do any extra cutting. That might mean the final wall sizes might be something like 9'10" x 11'9" (for example), but you won't end up with a lot of odd size scrap pieces left over that are 2" too short to use anywhere. If you have the room to make it taller, you can also add a couple of rafters up top to store really long bulky items to conserve floor space.

If you plan on collecting water off the roof you'll probably need a certain type of roofing material, then you also have to install gutters. Shouldn't be too hard, but will probably add more cost.

Also, unless you build it out of brick/concrete, there's only so much you can do to make it secure. Anything more than a padlock or two on the door, and thieves will just cut through the wall to get it in. I'd skip the windows if this is just for storage, your stuff doesn't need to see out, and you don't want people seeing in. Plus it will take up valuable wall space inside if you want to install shelves or stack boxes. If you alredy have an idea of what you are going to store in it, you can build shelves into the wall during construction.