This first triangle will form the basis of all the other triangles that make the Christmas tree.
Click on the Free Select Tool in the Tools palette. Now place your cursor on the point where the two guides cross and click to place the first point of the selection. Now move the cursor to the left and click once more near to the edge of the page, on the horizontal guide, to place the second point. The third point is placed on the vertical guide and the distance between the first and third points should be less than between the first and second. The picture will help show you what you are trying to achieve. Finally click the first point again to complete the selection.

