TL;DR at bottom
For probability questions like this, you can make simple fractions. The total number of objects would be the denominator, and the group of objects you have a chance to pick out would be the numerator.
We know that there are 24 total shirts, 12 red, 4 navy, and 8 white. Therefore, the chance of each would be 12/24, or 1/2 for red, 4/24, or 1/6 for navy, and 8/24, or 1/3 for white. Therefore:
#1: 1/2 or 50%
#2: 2/3 or ~66.6% (1 - chance of white shirts)
#3: 0%, no green shirts in the basket