How can I use my iPhone to record music the most effectively?

Sorry for the weird topic question. I'm new to recording music like this and just need some guidance. I have a full size MIDI compatible Yamaha keyboard, and love how garageband works. I understand that there are ways to connect these two (like with an iRig). But is there also ways to plug in, say, a guitar to my phone (because I noticed an amp section in garageband) so I don't use the not-so-stellar mic on the phone to record? Thanks for any advice.