I have a fast metabolism so I have an underweight bmi even thought I eat alot and am not very active what can I do to gain weight quickly
Caloric surplus is essential. In order to find out your consumption, you have to calculate your basal metabolic rate , use Mifflin-St Jeor equation given as:
X = 10*m + 6,25*h - 5*a + 5
Where m is weight (in kg), h is height in cm, a is age
Total energy expenditure is then given by a X*S ,
where S is coeficient of daily average activity, insert for S number approx 1,2 .
Resulting number is your daily calories consumption. Try to achieve daily calories ABOVE that number, roughly 300-500 surplus, this is essential for gaining weight. You can do it the healthy way btw