Can you please do a search for those questions first. They have been discusssed and asnwered numerous times here on the forums. If you still have a question about a specific step that doesn't work or you don't understand, then we are happy to help you.
Regarding your first question. GarageBand for iPad is a separate iOS application that you purchase on the iTunes app store. That app will run on all your iOS devices (iPad, iPhone, iPod). Once you have opened the app, tap on the Settings button (the wrechn icon) and at the bottom of the popup window is a help button. That contains the online help menu (you need to be online for that). This interactive online menu is pretty helpful to learn and understand GarageBand. And as I said, if you don't understand something about it, ask the specific question on this forum.Hope that helps
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How do I put garage band on iPhone 5 with the iCloud
To put GarageBand on your iPhone you buy the iPhone version from the iTunes Store; either directly on your iPhone or in iTunes on your Mac and then sync it to your Mac like all other apps.
Your iLife GarageBand version from your Mac will not run on your iPhone; both are very different apps and need different operating systems.
and how it downloads to the iCloud and the iTunes!
To download the GarageBand for iPhone songs to iTunes on your Mac you have several options - use the "Share" menu in GarageBand.
See: Share GarageBand songs, the paragraph on "Send a GarageBand Song to iTunes".
You can import GarageBand songs from your iPhone to GarageBand on your Mac, but not the other way round.