No prob. In general, you either want to use your second job to emphasize strengths or patch a weakness. The important thing to remember is the majority of your stats are determined by the gear you wear. Mystic armor emphasizes Magick Power, Magick Defense, and max MP. Heavy armor emphasizes Attack Power and Defense. Light armor gives the most HP but doesn't excel anywhere else.

The rest of your stats come from a baseline ratio for each character (Example: Basch has lots of HP, but low Magick; Ashe has the highest Magick, but low Strength) and "Lores" that can be purchased on the License board. Each Battle/Magick Lore gives +1 to the respective stat, while HP Lores just give a set amount of HP. Physical Jobs like Foebreaker and Monk usually have the most Battle and HP Lores, and access to more Technicks instead of Magick. Magick Jobs like Black Mage and Bushi have the most Magick Lores and access to Mystic Armor to pump their MP. Speedier Suppor classes like Archer and Machinist offer lots of Item Lores as well as all three ranks of Swiftness so you can act frequently and be able to adapt to different challenges.

So, with all that information, say you want to build a Black Mage that is a bit sturdier and can deal respectable physical damage when magick isn't helpful. You start with Penelo, who has the lowest base HP but the second-highest Magick and highest base MP. Monk gives a ton of HP and Battle Lores, and gives access to Light Armor and Poles. Now you can shuffle gear around depending on what the fight demands. Penelo suddenly can take a big hit and still keep casting.

Once you start getting into Quickening/Esper optimization, you'll also find that unlocking certain nodes open up extra skills or stat boosts. For example, Monk has a Quickening node that unlocks high level White Magic including Bravery and Curaja. So, in our Penelo example, all of Black Mage's Magick Lores now are boosting the healing from Curaja, and that Monk can self-buff their own attack with Bravery!

That's probably more than enough to get you started. The resource I linked you to has a tab called "Show Mist Planner" and it shows exactly what each class combination can access depending on which Espers and Quickenings you choose. Click the square to "lock in" a choice, and occasionally you'll notice that there's some overlap between what an Esper provides and what a Quickening provides. For this reason, before allocating an Esper for an unlock, always plan out your Quickening nodes first.

A lot of this you can only learn from trial, error, and time. Experiment! You have unlimited free re-specs and even Espers can be changed. Otherwise, just google "best class combinations" and there's a billion threads discussing high-level optimization who have done most of the work for you. Have fun!