How do I make a model/part go with a conveyor belt?

Make sure that the part you're trying to move on the conveyor is not anchored. Plus, you don't need a script to make a conveyor; just look at the Properties of the part that you want to be the conveyor under Velocity. (Don't worry about RotVelocity; that's for Motors.) Dimensions X and Z cause a movement on the part's top surface to go along the surface, which is what you would need for a conveyor. Dimension Y offers an upward velocity, which is what many people use for trampolines. The higher the number for a set dimension's velocity, the greater the velocity and the faster the movement of that velocity. In terms of making a part move along a conveyor (if you are making a tycoon, which I'm assuming you are), just make sure it isn't anchored. You don't need any fancy script to make it move along that conveyor, in all honesty. Your script,

while true do
script.Parent.Velocity = script.Parent.CFrame.lookVector *script.Parent.Speed.Value
wait(0.1)
end

doesn't really have to make a part's top surface have velocity. Just edit it under Properties. It's really that simple.