To the original poster,
If you feel you are ready to take it after your sophomore year is over and before your junior go ahead and take it then. I know a girl that did just that last summer, and got a 32P on her MCAT. She also has a real high GPA and good EC's.
However, if you are not ready and feel you could take a few more classes to prepare you for it like anatomy and physiology which will help and biochem which will help, then take it your junior year spring. If you absolutely cannot take it either of those times, then take it your summer after junior year; but beware taking it aug of junior year is asking for trouble.
You start filling your application, generally, in May right after your junior year is finished. Then you send it out by June or July ideally, after you have completed it.
If you send it early, you will get it processed quicker and get secondaries by July or August, and possibly interviews by early fall if they like you.