I am relatively new to blender and I want to make a triangle shaped plane in blender. Not a pyramid, just a plane shaped like a triangle. Thanks in advance
If you want to create equilateral triangle fast you may do it:
USING A CONE
Add a cone and set its vertices number to 3 via the Tool Shelf (T). Then delete the very top vertex with X.
USING MESH EXTRA OBJECTS ADD ON (REGULAR SOLID)
Activate the Extra Objects add on via User Preferences (Ctrl+Alt+U). Then go to Mesh-->Math Function-->Regular Solid. Delete the top vertex finally.
You may also use the Spin tool in Edit Mode and rotate a single edge around one of its vertices. Make the 3D cursor pivot for that, set Steps to 1, give any Angle and you're done. Still, if you want an equilateral triangle set Angle to 60. By analogy, it's 90 degrees for the right triangle.