Can someone PLEASE HELP ME WITH MATH :(!?

I have a set of numbers its 50 total i have to make a box and whisker plot but how do you do it? Someone said to take the two middle numbers add them and divide by two and thats the median and the same for the lower and upper quartile for the box and whisker plot but i don't know if thats how you do it or not i tried it and came up with 1978.5 for lower quartile 1992 for median and 2002.5 for upper quartile.