Probability first is a woman: P( 1W)

The probability that the first person in the line is a woman is 1/2 or 0.5 or 50%.
To find the probability of the alternate by gender:
Find the probability that the second person is a man P(2M). Knowing that the first person is a woman.
In this case the number of persons line up is 5 (as you don't need to count the first person):

find the probability that the third person is a woman P(3W):

Find the probability that the fourth person is a man:

find the probability that the fifth person is a woman:

Find the probability that the sixth person is a man:

Then, multiply all six probabilities:

Then, the probbility taht the first person is a woman and then alternate by gender is 1/20 or 0.05 or 5%