Choose a simple iOS app and download it from App Store.
You must have a jailbreaked idevice or you cannot dump the decrypted executable file from memory.
here are some tools suggestion:
Disassemblers: IDA Pro, Hopper Disassembler
I suggest using hopper, it could modify the code and save it back to .ipa. By the way, it is cheaper than IDA.
Runtime instrumentation: Cycript, Frida
Tweaks/Packages: OpenSSH, usbmuxd, libimobiledevice, iFile or Filza(Option), AppSync Unified