First of all, it's easiest to cook a breast of chicken that is even. If you have a large breast of chicken that is quite thick at one end and very thin at the other, I'd grab a little wax paper and a meat mallet or rolling pin and just pound it out a little to make it a little more uniform. This will help you to cook the breast evenly without one end finishing and drying out way before the other end is done.
Bring a little oil, butter, spray or whatever you'd like to heat over high heat in a skillet. Add the chicken once the oil starts to shimmer and smoke a little. You should hear an immediate pop and sizzle (psshhhhh) when you add the chicken. Now quickly turn the heat down to low and let cook for about 5-7 minutes. BUT!! The most important thing you are looking for, as pictured above, is to get down and look at the side of the chicken. Once it is white, and cooked through halfway up the chicken, flip it over. Cook an additional 5-7 minutes, and the rest of the white should rise up and meet the already cooked portion. When there is no longer a pink rim around the edges of the chicken it's thoroughly cooked and you can remove it to rest on a cutting board for 1-3 minutes to redistribute juices.
Think two halves make a whole, once both whites have met in the middle you know that no pink remains.
You can also touch the chicken, and when there is no longer a jiggle you know it's done.
Trust yourself, everything takes practice, but the more you do a little finger prod and watch the white meat touch you will learn exactly when your chicken is cooked and it will be far more moist and flavorful.