To find the least common denominator, first you should do a prime factorization on each denominator you are comparing.
That may sound scary, but it's not too bad.
24 = 2*2*2*3
12 = 2*2*3
28 = 2*2*7
The least common denominator can be found by choosing a set of prime factors that includes all of the above numbers.
In this case, choose 2*2*2*3*7, because each of the prime factors above are included in this set of numbers.
Then just multiply 2*2*2*3*7 = 168
168 is the least common denominator - it's the number that all the other denominators will divide into without a remainder.
Please comment if you need more explanation, or if you're not familiar with prime factorization.
Hope it helps!