I am CompTIA A+ certified.
The test should be relatively easy if you know enough to diagnose and repair a broken machine (and peripherals, and basic networking issues).
If you are planning to learn on the job, you will likely need to study up a bit. I walked in and took the test after having worked repairing computers for 5 years (and doing it for fun/free prior for quite a bit longer) and got a near perfect score on both tests.
Depending on what practice info you had, there is a good chance it is a little outdated. You definitely have to know some legacy stuff to pass.
I would recommend Professor Messer's free online video classes for all the CompTIA tests. His videos are very thorough, and he does a great job explaining things for the novice. (
http://www.professormesser.com/)
In my opinion, if you watch the videos all the way through for A+ once, you should be able to walk in and take the test.
When it comes time to take the test, be sure to search around for deals or vouchers, I save $120 by buying from a 3rd party and redeeming the vouchers on the registration website.
PM me if you have specific questions about my experience.