Balancing equations is more of an algorithm than a precise process most of the time. You try one thing, it doesn't work, you try another. It's the way some things go.
To begin with, its helpful to know the amounts of things on each side, how much carbon and hydrogen is reacted and formed afterwards.
On the left we have 6 carbon and 8 hydrogen, while on the right we have 6 carbon and 12 hydrogen. You want all the numbers for each element to be the same. Carbon is already there, six on both sides, it's just hydrogen that you have to sort out.
You need four more hydrogen to make 8 into 12, preferably without changing the number of carbons, though sometimes this is unavoidable and you have to do a few extra steps to sort this new problem, or try something else.
Luckily you have some elemental hydrogen, H2, which means you can change the number of hydrogen without affecting anything else. When balancing equations, elemental molecules are your friend.
To get 4 more hydrogen atoms, you need 2 more H2 molecules, which makes 3 in total, rendering the whole equation
C6H6+3H2→C6H12
There is 6 carbon and 12 hydrogen on the left, and 6 carbon and 12 hydrogen on the right. The equation is balanced.
