what you want to spend.
You can go expensive hundreds, even thousands. Or, you can go relatively cheap -- $0 by using the internal mic in your iPad, or a little more by purchasing a microphone and adapters to go through the audio jack. A better quality alternative, and more expensive, is going through lightning input (might need an adapter). I'm assuming the latter is what you are planning.
You can use the Shure 58 (or similar 57) mentioned above. It is a pretty good mic that is relatively cheap ($60 or so used). If you've been to just about any concert -- rock, folk, rap, country, etc. -- you've likely seen them used. But, you are probably going to need some kind of interface with that mic.
There are mics that go directly into the lighting input, having the interface and preamp to boost the signal built into the mic base. You can get something for $40 or so, but the better alternatives are $100 to $200. Buy used on ebay, and you can get your money back if you decide to upgrade at some point.
Here's one video explaining some of those mics (skip to about 5 minutes).
There are all kinds of videos and web pages that will help, but unless you've recorded using external mics before, it can be really confusing.
If it were me, I'd go to a Guitar Center and talk to the pro-audio folks (not has highbrow as it sounds), telling them what you want to do. If you don't have one near you, call Sweetwater. They'll assign a sales person (they call em an engineer), who can help you decide exactly what you need. Here's a link to Sweetwater where you can chat with an engineer/salesperson about lightning mics --
https://www.sweetwater.com/store/search.php?s=lightning+microphones&Go=Search
If you are willing to spend $200 or so, there is a well though of mic from Apogee especially made for iPad recording. But you can get by for less. The Shure 57 or 58 (the 58 is usually though of as a vocal mic, although both will work) will work if you get the right kind of interface to get it into your iPad.
Good luck, it's a lot of fun.