Sounds like an ambitious plan, but I have seen a lot people start small and make it happen when I comes to aggregating create content sources and putting together an ad network. Here are some tips to make it happen.
1. Get in touch with a blog/portal/community site/YouTube channel owners, basically content creators, (if you know the portal owner/s personally even better) and offer them a deal to get advertisers for their site and fix up your commission out of the advertising fee.
2. Find out brands/agencies/media outlets that do one time tactical campaigns with that portal which would in the form on advertisement or content marketing and cut a good deal for them since that is your first campaign, over deliver on the campaign expectations
3. Take this example and tie up with more blogs and portals in a same category to give a bigger reach for the next campaign
4. Get in touch with more advertisers and pitch your offering to them
5. Venture in more categories with the same process to expand you ad network!