DCL is a different animal than WDW for sure.
DCL has consistent pricing so going through a
travel agency (including costco) will NOT change the price of a fare. What might be different is the on board credit offered by many travel agencies including the board's sponsor dreamsunilmited.com or cash back in the form of a gift card to big box costco.
DCL is not going to give you a discount for multiple cabins so you'll have to decide what works for you and your family best (ie one person arranging or individual travel groups). I have arranged a family vacation doing all the work in tandem with a travel agent and it worked best for our family but everyone is different.
Best time to book-pricewise is opening day which is staggered based on loyalty level so being a newbie that would be on the General Public opening day for you. This occurred already for Jan-May 2019 so depending on the cruise you may or may not get the best pricing. Pricing on DCL works like this-there are a certain number of cabins in a certain category that are the lowest price. When those are reserved, then the next batch of cabins are sold for the second lowest price, and so on. You can see that if a cruise is super popular (example 2019 Panama Canal) the prices jump up fairly quickly.
This is how I would approach planning:
1) When? When school is out (breaks, summer, holidays) DCL prices are higher. During Sept/Oct/parts of Nov and Dec, Jan they are lower. Those obviously have downsides (think storms, cold weather, etc).
2) Where? DCL usually has more options in the summer, with the Magic ported in Europe and Wonder on the west coast and Alaska. Fantasy and Dream are in the Caribbean, doing 7 days and 4 day trips, respectively (*usually, there are exceptions with special itineraries).
3) How long? DCL has a few, short 2 day trips out of the west coast but the most common options are 3, 4, or 7 night options. Again, there are other amounts but deciding "short" or "long" is helpful.
4) What kind of cabin? This basically breaks down into inside (no windows, inside, cheapest), ocean view (windows), verandah (outside patio) and concierge (verandah, suites in some categories, and other perks). Those big categories are broken down further by location (fore, mid, aft) and deck.
5) Budget- how much you want to pay. There are wide ranges so I'd recommend you go to the DCL website and play with numbers to get an idea of the very wide price variations (this will help you possibly avoid sticker shock).
There used to be a good book- Passporter's guide to DCL but I believe they no longer print this. It is available in e book form from their website for something like $15. I used an earlier version when I was switching from the WDW vacations to the DCL focus and they were very helpful. Other guidebooks like Birnbaum were so-so.
Honestly, these boards are probably your best guide so don't be afraid to read and ask questions.