I'm an 18 year old female. is 140/68 a normal blood pressure??

It's a little high: Slightly higher than I'd expect for someone your age but we can't make much out of one reading. Check it a bit more often and if it remains that high talk with your doc about lifestyle (not medication) options first like diet exercise and cessation of alcohol (if you're drinking any)

BP: Blood pressure varies through the day, as a one time measurement, the 140/68 is fine, if the 140 goes further up during the day or if it stays at 140 systolic for much of the day then that would be concerning at 18 as its the upper limits of normal

High blood pressure: This is abnormal B/P . Was that the 1st time to have elevated B/ P. If so you need to moniter it regularly with your 1ry care physician. If persistent you need to have some work up, go on low salt & low fat diet & consult a cardiologist. Good luck