Formy, start with Fire Emblem Awakening since it's basically the best game for beginners to play, and make sure you go with Casual when you start, since you do get to keep most of your units, even after they "die" in a chapter. In previous games, when a unit dies, it's permanent, so you may end up having to soft reset and start a chapter over to prevent them from happening. I usually end up losing a few units when I played one of the older games, either because I couldn't protect them, or I just didn't want to bother resetting a chapter.