I would focus on building (or buying if you have the resources available to you) a couple properties with decent traction (enough traffic/downloads that you can run timely experiments with statistically significant sample sizes).

Once you're running a reasonably popular site (or app) every ad company, and their minions, will come after you and tell you why their platform is the hottest/best way for you to earn ad revs. You can learn a lot during this process (how their businesses work, who the key players are, etc). While this is happening you can also build and refine your adTech product and use the data from your site/app to prove why your new solution is the best thing going.

So in other words, BE the customer for awhile, the rest will fall into place.

the adtech ecosystem is pretty massive - for the generic (and even more confusing) breakdown of the value chain, check this out:

http://www.lumapartners.com/resource-center/lumascapes-2/

So where to start? it really depends on your business, will you have direct relationships with publishers? if so how will you do outreach to them + onboard them - if not will you get distribution on an exchange (if so this is capital intensive and there are usually min monthly buys $10-20k+), in which case you will then need advertisers (how will you acquire them?) - lots of questions pop out here

What's an exchange vs a marketplace? Whenever an ad loads on a web page or in an app (this is called an impression) a bid is placed by various entities in an auction and who ever wins shows their ad - this is an exchange

A marketplace is usually geared towards advertisers looking to purchase placements on publisher sites/apps - the marketplace has the rate info + ad format/size/specs and makes it easier to purchase from a large number of publishers in a curated way without having to go talk with each publisher directly.

How does a small startup get involved? it really depends on if you have funding or not, but adtech is one of the few places that make it easy to bootstrap

Shoot me an e-mail (in my profile) if you have any other specific questions, i have worked on the vendor and agency side.

> So where to start?

Network! Run a sort on Linkedin advanced search for Directors/VPs of Sales in this space. Put together a Top 10 hit list of targets. Reach out and engage in a phone conversation. Why? Sales guys are usually responsive and receptive to these kinds of conversations. They are also well connected to other helpful people. Bonus, it's an opportunity to pre-screen potential talent for your startup.

Make a list of startups in the space, and email a bunch of people in each, asking for advice.

Good luck!

The good thing about the ad space is that you can often find a niche = 1 particular feature that someone wants that you could build.

The bad thing is that it is very complex and that you'll likely need to spend time on sales.