I'll suggest, even though it's there for you, using a travel agent on Post is not necessarily the best move for you.
Use of a travel agent has no cost to you! Since it's your first cruise I'd highly recommend using a cruise specialist, rather than a general travel agent. An experienced cruise specialist can find and offer the same deals, and can perhaps offer you more guidance.
Once booked, if you choose, you can indeed book your shore excursions online at the Carnival web site. For a first time cruiser, with family, depending on the island, I do in fact recommending booking ship's shore excursions, rather than going independantly.
Except for the casino, all your onboard expenses will be billed to your shipboard account (which would be charged to your credit card, or you could pay off in cash before the end of the cruise). So, the question of how much money you need, would vary if you're talking about total expenses, or just necessary cash to have with you.
I also highly recommend reading many of the articles accessed from our Home Page. Many of them are designed for first time cruisers, and will offer a lot of information on things you may not have even thought of yet.