To find the average, you add together all the numbers and then divide by the amount that there is.
So, in your example, you have the numbers 8, 8, 10, 10, 10, 11, 11, 11, 11, 12 and 12. Adding these makes 114. There are 11 numbers, so divide 114 by this to produce an average of 10.36363636..., or 10.4 when rounded.
If a system like yours gets a lot of votes, there's a trick you can do to make the calculating simpler. Instead of adding up all the votes individually, you can multiply each score by the amount of votes for that score and then add all of those totals.
Using your example again, there were 2 votes for 8, 3 for 10, 4 for 11 and 2 for 12.
So you can do (2x8)+(3x10)+(4x11)+(2x12) = 114.
You don't really need to right now, but if you suddenly start getting lots of votes that technique could be very handy. And as before, you'd divide that total by the number of votes to get the average.