If you're a newbie to sewing, you might do better to use a pattern. I'm NOT trying to be offensive to your newbie-ness AT ALL, but adjusting sleeves and taking in armholes AND making the end result look nice is nowhere as easy as it sounds (spoken from having sewn/adjusted clothes for 43 years.)
If you don't want to use a pattern, then you might consider making a seam down the front and back of the shirt from the middle of the shoulder seam, passing over your nipple, and then going straight down to the hem (this is a form of princess seam).
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I've seen this pattern http://www.pamelaspatterns.com/Store_ProductDetail.aspx?pid=49EFB43A6CBE2F24 highly, highly recommended, although I don't personally know anyone who's used it.
If you don't want to use a pattern, then you might consider making a seam down the front and back of the shirt from the middle of the shoulder seam, passing over your nipple, and then going straight down to the hem (this is a form of princess seam).