In the Hall of Forms, you will be able to improve the first Forma unit that defeats a foe on any given map. The process of improving a Forma unit is called "shaping," and will occur once the map is cleared. Shaping can be used to learn skills or perform merges. However, be aware that there's no way of knowing ahead of time what skills or seals will be chosen as candidates for you to add to the Forma.
When shaping a Forma, you should typically look to prioritize weapon skills and Special Skills for an immediate boost to fighting strength. That said, you should still keep an eye out for valuable Passive Skills when they appear, such as Atk/Spd Push 4 as an A Skill, Lull Atk/Def 3 as a B Skill, Time's Pulse 3 as a C Skill, and so on.
When starting out, a Forma unit will be at a low level and it won't have any skills. During this time, the best approach is to gradually strengthen the Forma by defeating enemies in lower Chambers. Don't rush to advance to higher Chambers—the Chambers that you've already cleared can be great places to gain EXP.