1) On the ribbon go to the View tab. Under the Workspace panel click the Windows dropdown and enable Measure Tools.
2) On the Measure Tools pane click Options... (or hit F12) and go to Interface -> Snapping and enable Snap to Vertex, Snap to Edge, and Snap to Line Vertex. Click Help if you need more info on each of those.
3) While in the options menu (F12) also go to Interface -> Display Units and make sure this is set to the accuracy and format you need. To have it display like "6ft 4in 1/8" you should put the Linear Units setting to Feet and Inches Fractions. Leave Angular Units on the default setting unless you have a need to change it, and set Decimal Places and Fractional Display Precision to whatever you need.
Once that is done go back to the Measure Tools pane that you opened and click the very first icon for Point to Point (it's a diagonal line with a circle at each end) and then click your first point to measure and then your second point. The 3rd from the last icon (ruler with a red circle and an X in it) will erase the measurement you just took, or you can just start a new measurement.
Since you're just starting to use it you should measure some stuff that you're 100% sure you know the dimension of already (like a box that is 5'-6" x 4" x 2" or something). This will help you figure out how the snaps work and how the icon changes when you're using it