There are plenty of kit-set type sheds out there, or you could even be considering building your own from scratch. Either way, there are a couple things to keep in mind that could be the difference between a good shed and a great one.
As with anything, laying a good foundation is the key to success. Some sheds are supplied with flooring material, but most aren't, leaving you to your own devices. If your shed is any bigger than a simple shelter for the lawnmower, you'll want a proper floor to keep the structure stable. An appropriate base will also prevent things like water, weeds, slugs and mud from getting into all of your tools and equipment.
For this reason, we would recommend laying down a concrete pad for your shed. If you haven't poured a pad before, make sure to get some advice from the crew at your local Kennards Hire - it's a pretty big job, but it's well worth it. Pouring concrete is a valuable skill for the DIY enthusiast and will come in handy more than just this once, so learning how to do it properly is an investment in yourself.
A concrete pad of significant size is more than just cement, sand and water. You will also need coarse fill for the basecourse, some sand in its own layer and some polythene before you even reach the concrete. And somewhere in the middle of your concrete mix you'll want some wire mesh for a bit of extra rigidity.
For smaller sheds where you'll probably just use pavers to support your shed and keep your tools and workspace dry, don't put your pavers straight on the ground. The surface is probably fairly even, but fairly isn't perfect. Instead, spread some gravel around and use a screed to get it just right. This way you can start on a flat surface. You're also more likely to have water effectively drain away from the shed itself.
There other kinds of layers too - think about your walls. There are many ways you can improve the standard walls on a shed. If you plan on working in there in the winter, some insulation and gib board wouldn't hurt. Windows are great if you need sunlight to work by, but these should be reserved for larger sheds, as the complications they add to minor constructions is not worth it most of the time.