Good reads:
A) FAQ / Beginner's Guide
B) Class and Army Guide

Yes, best way to level up units below 50 is feeding them all iron and bronze trash. If you are running dry on money you can sell irons, but at the beginning leveling the core should take priority. Just feed them as they are, they are plentiful anyway. Advancement of story missions and tutorial / urgent mission practice missions gives you essential silver units for drop completion, so it is a good idea to push as far as you can)
Commonly, first units you want to level besides her are Archers (Bashira is OP for a free unit, then either Soma if you don't want to bother much with affection farming, or Daniela if you want more damage rather than utility and not scared of cows XD), Healers (your free golden Iris would be a good first Healer to raise), Heavy Armor (Leanne will serve you well deep into the mid-game) and Mage. If your account rerolling / starting premium roll managed to net you one of the cheat blacks, add them to the list, but otherwise stick to raising the core army first.

Also, this is not, bu any measure, a long post. Worry not.