Since this is your first cruise, the best advice I can offer is to definitely not book it directly through the cruise line or an online site. Use a travel agent, preferably a Cruise Specialist. Their services can be invaluable, especially if any problems occur. But if they start recommending, then run the other way. Their job is to provide you information so you can make a educated decision. It's not their job to guide you into something they want.
As for what, where, and when, it really depends on what you want, where you want to go, your lifestyle, and of course, your budget.
Keep in mind that what one person loves, another will hate. So take recommendations with a grain of salt.
With that said, if you want a more laid-back cruise, then I would suggest taking a long look at Celebrity and then Holland America. Both attract an older crowd, but as I like to tell people, they're for the people who appreciate the finer things in life without wanting to pay for them. I personally rate Celebrity just a little bit higher than Holland, but both are very nice, offer very good food, and wonderful service.
If you want a more intimate experience, the smaller ships are a great experience. They larger ships will have more amenities, but also more people. However, it's really tough to beat the new Celebrity Solstice-class ships - they are absolutely wonderful!
As for an oceanview cabin, almost none of them have portholes anymore. Sometimes the cabins in the very front or very back will have some, but the majority of them are large picture windows. And NONE of the portholes open. As for a balcony cabin, obviously those are the nicest as you can sit out on your own private balcony and watch the world sail by. If budget is an issue, then this may not be an option for you.
As for the destination, the best thing to do is talk to your agent so they can talk about the various ports of call, which could help you, provided they have personally been to those ports. There are so many options, you really need to decide where you'd like to go. Make a list of places you'd like to see and then a list of places you don't want to visit, then give this to your agent. When deciding locations, again, don't listen to what alot of people will tell you. For example, I hear from people all the time that say they don't want to go to Jamaica. When I ask them why, they always say the same thing, 'my friend told me....' Again, make up your own mind. We've been to Jamaica on many occasions and have always had a wonderful time.
As for when to go, again, this depends on you. The best prices will normally be in September, November, and December. People want to travel on the holidays, so you'll find much higher prices and alot more kids. But people don't want to travel between the holidays. So the first/second week in December will see some great deals.
As to when to book, I always tell people the same thing; as soon as possible! That may very well be the best rate and rates could go up. If you've already booked, then you're protected against any price increases. And if you've booked and the prices go down, your agent can get the lower rate for you provided final payment has not yet been made. And definitely do not listen to these misleading sales pitches about 'save up to 75% on last minute cruises'. Simply stated, it's false advertising. All the major cruise lines require everyone to sell the same cruise for the same price. So find a good agent (again, it's best to get a good Cruise Specialist), and work with them to find you the best cruise for you.