Forma units will start off with only a single weapon skill, so it's a good idea to prioritize having them learn skills for each slot before all else. However, bear in mind that only the first Forma to defeat an enemy during a battle will be eligible for shaping.
While shaping a Forma, the unit can learn from a pool of seven types of skills: Weapon Skills, Assist Skills, Special Skills, A, B, and C Skills, and Sacred Seals. You can replace any of these if the Forma has already learned a skill of the same type, in which case the previously learned skill will be overwritten and lost.