Women’s promise rings can basically be worn on any finger you want, however, you should take into consideration whether this is a promise ring or a pre-engagement ring?
If so you can wear it on the ring finger (the fourth finger) of your left hand as a placeholder to be replaced in the future by an engagement ring. Do keep in mind that if you place it on your left ring finger you will need to switch fingers for it when you do replace with an engagement ring, that might mean you will need to resize your promise ring as not all fingers are equally sized even if you just want to switch it to your right-hand ring finger.
Promise ring worn on the left hand
On the other hand, if you want to be sure it won’t be mistaken for an engagement ring and avoid some awkward discussions around it – stay clear of this finger, you can choose any other finger to your liken.