I will go to an Enduro race and I want to take sharp, in focus, photos. I only have a Canon 550D, EF-S 18-55 IS and EF 50mm f/1.8. I tried using autofocus on moving motorcycles and 90% of them were out of focus.
I have not tried motorcycle events but have successfully photographed Ice Hockey matches with reasonable success so this advice is based on that.
Set the camera to shutter priority (Tv) and start with a minimum of 1/250. You may can go faster if the light permits. The smaller the aperture you can get (while keeping the 1/250 min) the better to maximise the depth of field and ensure you capture the subject. (Unless you are aiming for bokeh )
You will also need to experiment with what focus mode is going to work for you. You should have an AF Servo mode which tries to track focus on a moving object however your lens may not focus fast enough to keep up with something like a motorcycle.
An alternative then is to set to One Shot AF, find a point that the subject is passing and fix the focus on that (press the shutter halfway) and wait for your subject to come into frame. As you get a feel for it you can try tracking with the subject until it passes your preset focus point before shooting.
In either case set the camera to continuous shot mode and take as many frames as the camera will capture to maximise your chances of a good one.
In addition to Kevin's answer -- fast shutter speed, experiment with AF modes -- here are three more tips that will help you reach success:
Use manual focus. The fastest autofocus systems are too slow for some uses or are fooled too easily for some uses. But manual focus lets you avoid those troubles! Prefocus where you know the motorcycle will be and as it comes into focus take the photo.
Practice. Practice. Practice. Practice. Practice. Also, practice will help immensely. Intimate familiarity with your camera will help you understand precisely what it's capable of and how to avoid its limitations and work its strengths.
Learn about enduro racing. The more familiar you are with the sport, the more easily you will be able to anticipate what will happen and where and when an exciting shot will present itself.
In motorports photography a lot of images are panned captures. This is how the wheels get blurred and show motion. You can do this with a slower autofocus system such as the one you are using.
Kevin's approach of focusing on a spot where a motorcycle will pass is very good advice. You can use manual focus or spot, whichever makes you feel more comfortable. Using shutter priority is also very good advice. The rule of thumb for panned images is you want to have the shutter speed at 1/mph. So if the bikes are traveling at 100mph you want a shutter speed close to 1/100 of a second. You can go up and down to practice as well. The lower the shutter speed the more accurate your panning has to be. The lower the shutter speed will also produce more motion blur and a pleasing image.
You won't need a large aperture in daylight because the motion blur will also blur your background. It does help if you can position yourself in a location where the background is mostly a single color. Something like the back of a hill that is all green or all brown. This will help produce a more pleasing background when you get a good panned shot. The smaller aperture, larger f(number), gives you are larger depth of field as well. This helps with your focal point since you will most likely be pre focusing on the spot where you want to snap the capture.
In my experience, I use a camera that can shoot 8 frames a second and I find it much more reliable to time the shot and click a single photo. It keeps my hands more level while panning with the subject. You can experiment with both approaches but for me the single shot produces many more keepers.
Panning is accomplished by following the subject in your viewfinder along the subjects path of travel. The goal is to capture the subject while keeping your camera moving and keeping the subject in your viewfinder during the capture. You want to keep your camera and lens level with the plane of the subject to keep the subject sharp but allow for motion blur in the wheels.