From the manual:
Order controls whether Anki should add new cards into the deck randomly, or in order. When you change this option, Anki will re-sort the decks using the current option group. One caveat with random order mode: if you review many of your new cards and then add more new cards, the newly added material is statistically more likely to appear than the previously remaining cards. To correct this, you can change the order to ordered mode and back again to force a re-sort.
So the “show new cards in random order” option is what you want 
Alternatively and for more control, you can reposition new cards from the browser. Please see Introduction - Anki Manual