One measuring instrument that is used a lot in the manufacturing industry is the micrometer. These are also very popular with machinists making quality pieces. In class we are using outside micrometers; this means that the instrument is intending to measure the outside dimensions of objects.
Here is a great reference on how to read the measurement from the instrument.
There are other resources available as well. As with so many things we do, practice makes all the difference in reading these instruments accurately.