I sell ads on my social network of 9 million+ to small businesses, but I want to sell higher priced ads to corporations. How do I start?

I would really need to see the ads that you're selling to be able to suggest what method to scale them with... but you hit the nail on the head...You have two options really...building internal sales processes or going through an agency. Usually companies go through an agency first, to get professional handholding in "sales product" development...ie. they help you scale your pricing and ad units and offerings...and get your foot in the door with bigger clients...but you can do a short term contract with most agencies...say 6 months...so that after 6 months you are in a much better spot to handle it on your own and stop paying 30% to an agency...

I'd recommend one of the many services you can sign up for that facilitate connecting influencers with companies:

https://famebit.com
http://www.revfluence.com/
https://advowire.com/
https://izea.com/
https://www.tapinfluence.com/influencers/

Here's a review of those, and many other services from the advertisers point of view (which will help you choose too):
http://www.business2community.com/brandviews/content-blvd/popular-influencer-marketing-platforms-reviewed-01549843#TiKjqjqwDkk6RBws.97

There are media companies you can use to get to the advertisers directly, through them. Flatiron Media comes to mind (http://flatironmedia.com/), I'm sure there are other. They have a roster of advertisers they buy media for from sites for their clients which are advertisers.