G.I. Joe always told me 'knowledge is half the battle'. So for the newbies in the hiz-ouse - If you're still shopping for a microphone to suit your DSLR audio needs, you'll probably already find that the Rode VideoMic is a very popular one. I've run my own tests and found that it does perform much better than some microphones going for twice it's price. But before you start throwing down a solid buck, you can read up some other interesting suggestions through the B&H Insights blog that might help you get a bit more 'knowledge' - Go Joe!.
So a microphone will help you focus in on a little better sound than just the onboard microphone, but for the best sound processing, you'll want to shop for a portable audio recorder. The in camera audio of the DSLR's are just not up to par for the true audiophile. The link below jumps you to another B&H Insights article about different Portable Audio recorder solutions.