We have a Project Guide that will walk you step by step through a science fair project using the scientific method.
Look at the sections under "How to Do a Science Fair Project" on this page: http://www.sciencebuddies.org/science-f ... ndex.shtml
You need to figure out a way to accurately measure how fast each brand pours. You could get some clear narrow tubes (all the same diameter). Then pour the same amount of ketchup in each tube (not all the way to the top, maybe just a third full).
Then use a marker to mark where the ketchup started. Make a second mark near the end of the tube (measure the distance to make sure it is the same for each tube). You could then put some tinfoil into the tube to keep the ketchup in place (don't squish it, just go to the mark). Then gently turn the cylinders upside down (they need to be held vertically by something). Slowly take out the tinfoil, then measure how long it takes for each ketchup to go to the second mark on the tube.
Another way to do this: instead of putting the tubes upside down, you could put a marble into each cylinder and see how long it takes for the marble to get to the bottom. You could do this with an ordinary clear glass (might be easier than finding narrow tubes, unless you have some already).
Hope that helps!
Let us know if you have more questions.