First, you should NEVER book a cruise with an online site. If you need assistance, you'll get someone who is not a certified travel agent, works in a call center, and has usually never even been on a cruise. Same thing is true with booking directly with the cruise line.
It's always best to book with an agent whom you can talk to, help you with every aspect of the cruise, answer all your questions, is a certified travel agent, and preferably someone who is a cruise specialist.
Keep in mind that most mass market cruise lines require everyone to sell the same cruise for the same price, so it make sense to utilize an agent because their services are free to use and they represent you should any problems arise.
With that said, it's obvious to me this agent has no idea what they're talking about, so I would definitely drop them and work with someone you can trust.
Here's how it works; when you book a cruise this far in advance, the cruise line will allow the agent to put a complimentary hold on a cabin. The amount of time they can do this will depend on how far out the cruise is. In your case, normally it's only about a week. You cannot put a complimentary hold on a cabin for a few months.
The amount of deposit will vary depending on the cruise line, the length of the cruise, and whether or not there are any specials going on that will allow a reduced deposit or not. Again, your agent will be able to provide all this information to you.
If you'll let me know the specific ship and sail date, I can provide more specific information.