If you earn a living from machining, you'll want to get something like a Mitutoyo - which will not be cheap, but should last you forever if cared for. If you are a hobby machinist, you may be satisfied with an import - take your pick on brands.
Note several things regardless of the brand:
The accuracy and precision (repeatability) of your readings will depend on developing a "feel" - it is possible to change the reading by half-a-thou by how firmly you turn it to the measurement. Two systems are used to try to give consistency of "feel" - a "friction" style and a "ratchet" style. There will be proponents of each - take your pick, and learn to use it against a standard so that you can get accurate and repeatable results.
But with regard to the standard, the second thing to know is that micrometers are adjustable. Generally there is a set screw (maybe under a cap) or some other mechanism to lock the barrel to the spindle. Once you have a consistent "feel" for how you use the micrometer, measure a standard, and if it is off, unlock the barrel, rotate to the correct reading, and lock it back. From time to time, check the micrometer against a standard to make sure it is still set correctly.
Finally, note that any debris, even tiny, can throw off the measurement. Make sure the part is clean, and make sure the ends of the spindle are clean as well.
