Triangles are one of those things that seem like they should be a fundamental part of any vector graphics program but sometimes don't appear in the menu. Like Adobe Illustrator, Inkscape classes triangles as polygons. So, to make a triangle you first select the star/polygon tool:
When you click on it you'll get a new menu at the top, you want the following settings:
It actually doesn't matter with a three sided shape whether you select star or polygon as your starting point, however, if you use star you'll need to set the spoke ratio to 0.5 otherwise you'll get additional unexpected corners in your shape. You don't want any rounding for a true triangle so set rounded to 0. If you want an equilateral triangle then set "Randomized" to zero. Pull out your triangle (holding down the ctrl key if you wish to align it horizontally/vertically).
Making the triangle have precise dimensions
Once you've drawn your triangle click on the Select tool:
This will change the handles on your triangle from this:
And will allow you to manipulate the width and height of the shape using the menu bar at the top:
If the lock is closed then changing either the width (W) or the height (H) will also change the other property. If it's open they can be changed independently. Click the lock to toggle between the two. If you click in either the width or height box you can type the dimension you wish the shape to have in, or you can use the up and down arrows on the right hand side to change the dimension by 0.1 of a unit (clicking in the box and using the up/down arrow keys will also change the value by a 0.1 of a unit). The units ( cm / mm / px / % / &c ) can be changed by clicking on the dropdown to the right of the width/height boxes.