You'll need the tube itself, a boat and harness or tow rope for towing it, an air pump to blow up the tube, and the necessary safety equipment. For the sake of safety you'll want to have life vests for anyone who plans to ride on the tube, and though many people don't use these you'll also want to consider water helmets. These are particularly useful when you have multiple riders because if two riders fall off the tube and hit their heads together they'll be much safer if they're wearing helmets.
Other than that equipment for safety the only other precautions to take are that everyone who rides on the tube is a strong swimmer in case they slip into the water and that you follow any posted regulations where you are going to be using your boat. These few simple precautions should ensure a safe, and fun, day for everyone.
These towable inflatables come in all different shapes and styles. The biggest difference is how many riders they can hold. You can get a tube for one rider, or a dozen. They come in all colors, various brand names, and varying styles. If you're looking for something just the kids consider one of the child sized options. You can get a normal circle shaped towable, or a variety of unique designs, such as a dolphin. There really is something for everyone, the options are endless.
These make great toys for a day at the lake and always impress those who haven't ridden before with how much fun they really are.