When you switch from an iPhone to a Galaxy it means that you will have to deal with two different systems and transfer all your data from iCloud to a Google account. It’s actually easier than it sounds (and pretty quick too)!
This video is really helpful and explains how it can be done by using a single app step by step:
As he said, it’s important to turn off iMessage on your iPHone as soon as possible before using your new phone, as you might not receive new texts because iMessage will try to incercept things sent to you from an iPhone.
Hope it helps!