It would seem that it could be easier?
However, for beginners this can be a real problem, since the figure "triangle" is not in the standard tools of Adobe Illustrator.
Method one. Select the Pen tool ([P] key) andClick in three places in the work area to get a triangular shape and the fourth time to the first point to close the outline. You can also close the outline by selecting three points and pressing [Ctrl + J].
Method of the second. Select the Line Segment tool ( key) andDraw three straight lines so that the end of the previous line coincides with the beginning of the next one (the end of the third line should coincide with the beginning of the first line). Select the resulting lines and press [Ctrl + J] to close the path.
Way the third. Select the Rectangle tool ([M] key) andDraw a rectangle. Next, select the Delete Anchor Point tool ([-] key) and click one of the corner points of the rectangle to delete it.
Method Four. Select the Polygon tool from the same menu,Where the Rectangle is located. Click on the work area and without releasing the left mouse button, you can press the up and down arrows on the keyboard to adjust the number of corners of the polygon.
Way Five. Select the Star tool from the same menu whereIs Polygon. Click on the work area and, without releasing the left mouse button, you can press the up and down arrows on the keyboard to adjust the number of star ends.