Anchoring a small outdoor storage sheds is a must, particularly when strong winds or violent storms occur. Failure to anchor in these conditions generally means your shed will move or in worse case situation simply fly away.
Metal sheds are designed to be anchored to a solid masonry or concrete slab, some even come with a concrete floor anchor kit including a channel clip and masonry bolts.

Storage-sheds anchor kit

If a concrete slab sounds too hard here is a tip for you, attach your shed to your house or fence. Simply securely attach some hardwood 3×2 pieces to the wall or fence and then attach the shed to these hardwood pieces. This method can be applied to resin or wooden sheds.
When attaching any shed in this way make sure the shed is placed on level ground.
Also adding a floor to the shed that is attached to the walls will, when loaded with goods, add extra weight to hold the storage shed in place.