Trips to Vegas or Hawaii can be arranged with a computer and a few clicks of a mouse, but safaris are NOT something you want to put together by yourself using just the computer. Safaris are complex affairs where a professional safari-planning specialist with first-hand experience in Africa is your best asset. He or she can help you navigate through the maze of choices and options so you end up with a holiday that suits your needs, activity level, comfort zone, and budget.
Of course you can narrow down your preferences in advance by doing some online (or offline) research: there are many printed reference guides and online resources that can help with this. For example SafariBookings.com allows you to compare different lodges, game parks and tours by reading professional and traveler reviews.
Once you have a good idea what kind of safari you want, find a safari specialist who caters to that particular need. Not all safari planners handle all regions and all types of trips. Some are experts in pricey luxury camps, while others know Uganda better than other countries.
When you pick a safari planner, make sure to ask pertinent questions: How long they have been in business? Have the staff have personally experienced the tours/lodges? Is airfare is included in the price? How large are the groups? Do they offer off-the-rack trips or bespoke journeys? Can they offer references/testimonials from previous clients?