You might find it helpful to print off this sheet
of coloured triangles and cut them out.
Can you begin to picture in your mind what the shape will look like?
Try joining two triangles together to start with, then add another one on, then another etc.
Don't be afraid to take apart what you've done to alter it if you discover something isn't quite right.
You could begin by joining the triangles together loosely with a small piece of sticky tape so you can easily cut them apart again if you find you want to change your mind.
will show some triangles with tabs on for joining in 3D.