How should I advertise the first item I'm selling on my website?

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

I'd probably start with social media marketing as a way to build interest in your product. You can run paid social ads, but that works best with a solid social media plan in place, one that covers multiple social networks.

From there you can run some pay-per-click ads directed to a landing page on your website. Once you get yourself established you should then move into a full-scale web marketing campaign.

You have two major challenges:
1. Driving traffic to your website.
2. Making them buy there.

A single SKU does not usually make an Internet business. What is the product? What is your current web traffic. who/where is it coming from, Is there already a want and desire for it in the market?

If you do not clearly know these answers, then you must rethink the whole process.

If you have a single product that is consumer oriented, plant seeds by testing on Amazon and/or eBay. The most female-skewed social media site is Pinterest if your product may sell via visuals.

Your question is screaming "I need a Business and Marketing Plan." You must start there.

It is easy to advertise your new product.
1. Send Free Samples to Influencers.
2. Start Blogging.
3. Build an Email List.
4. Sponsor an Event.
5. Interview Industry Influencers.
6. Pull a PR Stunt.
7. Experiment with AdWords.
8. Share the Load with Affiliate Marketing.
You can read more here: https://neilpatel.com/blog/your-first-online-sale/
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath