You should first figure out your audience more specifically.
Females 15-40 is very broad. Will all of that audience like the 1 product you have..?

Get specific before you advertise. For example - you are selling iphone cases so you would ideally like the ability to target only people with iphones.

Facebook seems a logical place to start as you can plug in your current profile of iphone users females ages 15-40.
Location - local national or international..?
(Im going to guess local & national)

Also you might want to consider the style of your cases will appeal to certain types of people more than others.

If you have a minions case (hot at the time of writing as the first minion movie is out)
This might currently have a broad appeal because of the movie.
So you can also add in this as an interest when you promote your cases on facebook.

Or if you are selling leather folding iphone cases there might be a different type of person who would be more interested.
It might be more of a male/female mix as well who are slightly older 30+ at a guess (of course I would need to research - this is purely for illustration)

My point hopefully is that before you start advertising you need to figure out who your advertising to and what specifically they will want.

Facebook is a great place to start as you have so many choices to only promote your products to exactly the people who are interested in what you are offering.

Hope this helps
Good Luck
Phil

ps.
If you need help with this I can offer a strategy session to help with this part of your business plan - this will help you build a solid base for you businesses long term success.
Call me