By default, your products are displayed in your store by creation order. You can edit the display order of these products.
To do this, go to the listing of your products (My shop / Products in the header of your admin area), click the "Manage products" above the table then click the "Sort products" button:
Click thumbnail of a product and drag it to the position where you want to assign it while holding the button. Release the button once you have reached the position of your choice.
You can also change the order of your collections in the same way.