One billion = 1,000,000,000 (a 1 and 9 zeros)
One trillion = 1,000,000,000,000 (a 1 and 12 zeros)
To multiply the two together:
1. multiply the two 1's together (which of course gives you 1).
2. Add together the total number of zeros, giving you 21.
Put the answer of 1 that you got in step 1. in front, then add the 21 zeros onto it, giving you
...which in words would be written as "one sextillion".
I'm using American numbering throughout because that seems to be turning into the default system throughout the world.