More practically, study the data kept at peeringdb.com, bgp.he.net, bgp4.as, radb.net, and ripe.net for concrete examples of what details are important.
Peerings may be unilateral (someone pays), bilateral (no one pays), or multilateral (everyone pays a little to exchange routes on a single route server). Most companies rely on unilateral peerings to gain direct access to the Googles and Microsofts of the world, and these tend to have certain requirements: either you must pay money, or your autonomous system (AS) must achieve some daily average of traffic bound for the peer AS. PeeringDB is a good resource for discovering these requirements before contacting potential peers.
I hope this helps. Best of luck to you in the travails of working with highly distributed, stateful systems you can't touch; it's a real shit show out here.