I guess I should expound on what online marketing is...
There's two paths: Selling products made by others as an affiliate or selling a product you made. These products are digital 99.9% of the time removing the need for storage, packaging and shipping. The usual process goes something like this:
Get a potential customer to a "squeeze" page somehow (this is a one-page website that offers something interesting, usually for free). The interested party needs to fill in their email address (and normally their first name) to get the free item. An auto-e-mailer will then add their email address to a subscriber list, email them back to confirm that they actually requested the item (with a link). When they click on the link, two things happen: One, they get another email with the free offering (which makes them happy and builds trust) and, two, they get sent to another squeeze page that talks about a related item. This new page is probably selling the item in the $27-$37 range. If the person decides not to buy (by closing their browser or going to a different page), it will stop them and ask them to stay. If they do, they usually get presented with a discount (or a similar, but cheaper item). If they buy, they are typically given a "one-time" chance to buy a much more involved (larger, or more valuable) program - this is one that is much more expensive. If they don't buy, they're still offered another related item that is less than the previous program, but more than the item they bought.
They are now on a subscriber list. They will automatically get emailed with encouragement, tips & advice (without any attempt to be sold something else) or other free items that they might be interested in (which starts the process again!). If they remain on the subscriber list, a degree of trust will have been built up so now other offerings could be recommended.
One example using Guitar Playing as a niche:
(Free offering) - A fretboard finger exercise book
(Next offering) - A book/video combination showing how to augment basic chords with new shapes and positions
(Main offering) - A full video course going over several aspects of playing guitar including future weekly videos (a flat fee up front followed by a monthly renewing subscription). This is called the "up-sell."
(Alternate offering) - A book/video combination showing how to choose a soloing key over the main key of a song. This is often called a "down-sell."
Additional free offerings might be a chord reference book or a quick guitar maintenance handbook. Then recommending courses for perfect pitch and relative pitch.
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So, this breaks down to these parts:
- Having decent free and valuable paid products (which, as mentioned above, could be yours or someone else's)
- Creating a small but enticing "squeeze" page
- Driving traffic to this squeeze page
- Having the behind the scenes stuff like a Clickbank account, an AWeber account, domains, web hosting, access to free stuff (like from Master Resale Rights), paid for pictures (like Shutterstock), social media accounts (if you go that way, things like Facebook, Twitter, Instagram, Pinterest, Reddit, etc.) and cheap help when needed (see Fiverr).