Mean - this is the average. Add up the numbers and divide by the count of the numbers in the data set.
11+13+14+16+17+18+20+20+21+24+25+29 = 228
There are 12 numbers, so divide by 12.
228/12 = 19. The mean is 19.
Median - - - this is the MIDDLE number. Your data is already in numerical order (hooray!) so there are 12 numbers. Since it's even, we'll take the average of the 2 middle numbers. The 6th number is 18 and the 7th number is 20. The average of these is 19. So the median is 19.
(Side note: yes, median and mean can be the same.)
Mode - - - this is repeat/most common numbers! 20 repeats. 20 is the mode.
Range - - - biggest number is 29, littlest is 11. The range is 29-11 = 18.