The best way to approach planning your trip is to start by visiting Disney World's website. Look at the 'special offers' section to see when discounts may be available. My family prefers to travel in the fall, and there are often discounts available for this time of year. Keep in mind that when the schools are out for Christmas, spring break, and summer break, that Disney doesn't have to offer discounts because these are their busy seasons.
I always recommend staying at a Disney resort, but of course staying 'offsite' is an option, too. I would never go this route because for me, part of the fun is never leaving Disney property. Transportation is included in the cost of your resort price, but if you stay offsite, you'll pay to get from the airport to your resort, and then you'll have to (more than likely) pay to park at the theme parks every day- keep this in mind when deciding where to stay! Daily parking can add up (currently it's $20 per day), and it may end up being less expensive to stay on-site where transportation is included!
While checking out the website, take a glance at the different ticket options as well. If you do go with a travel agent, she or he will help point you in the right direction, but it doesn't hurt to familiarize yourself with some basics first. While AAA is the official travel agent of Disney, I have actually clued them into different discounts which became available, so it never hurts to do your research!
So to recap: Figure out whether you'd like a travel agent, and if so, a bit of research to find one who's reputable is a must! Familiarize yourself with different discounts offered throughout the year, and decide whether you are able to take advantage of these offers. And finally, do some research regarding the different resorts and ticket options which are available to you. Happy researching!