By default items are shown in alphabetical order. You can override this alphabetical order by entering a view order for each item. The view order will be used to sort items, ie a view order of 1 would be shown before a view order of 2 and so on. If 2 items have the same view order a secondary alphabetical sort will be used.
To change the view order for an item please visit
Inventory-->Items-->View/edit. (New item pages)
Search for Item.
Search for Items.
Click on the Advance Edit link next to the item you wish to modify. In the edit item screen select the Options tab and then enter the view order in the box labelled view order and hit save. Please note that these are not actual positions but rather sort orders. For example if you had 3 items one with view order 10, one with view order 20 and one with view order 30 the view order 10 item would be shown first, the view order 20 item would be shown second and the view order 30 item would be shown 3rd.