I am 35 and was informed through ct scan that i have a "low density" 6mm lesion on my kidney that they believe is a cyst. i am really worried. are lesions on the kidney common? is low density a good thing?

Why was the CT done is more important. The unfortunate thing is that inconsequential findings from CT could result in more unnecessary tests, expenses and anxiety. It is most likely inconsequential, however discuss with your health care professional so that adequate follow up may help in making sure may be the best option.

Cysts are very common. Probably about half the people over the age of 50, will have at least one or more renal cysts. 6mm is pretty small, likely too small to tell with certainty if it was a cyst. The likelihood of it being something more serious is pretty low. Statistically it is most likely to be a renal cyst. I would not worry much. Have it followed up in a few months with CT or ultrasound.