Prime factorisation is breaking down a number into primes which mulitply to create that number.
To break down a number into primes, you can draw a factor tree. For example:
12
/ \
4 3
/ \
2 2
So 12 = 2 x 2 x 3, or 12= 2^2 x 3.
Alternatively, on a scientific calculator, you can find prime factors easily. Type your number then hit equals, then press the FACT key for factors.