Upgrading 2010 to 2013

I have been tasked with upgrading our current Farm. Can anyone point me in the right direction regarding how to migrate from 2010 to 2013? The only custom part of our farm is the Event Handler that creates document numbers for all our documents that get uploaded. So installing 2013 on a new box is fine but its the unknown of once installed what steps do I need to take before migrating our farm.

---

dfrancis