How many electrons would carbon nitrogen oxygen need to complete the second energy level

Electrons circle the nucleus in energy levels also known as electron shells.

First energy level can fit two
Second can fit 8
Third can fit 8
After that it gets more compicated but you don't need to worry about that here.

How do you know how many electrons an element has?
Look at a periodic table
It is the smaller number
In Carbons case this is 6, this means it has 6 protons in its nucleus and therefore 6 electrons in its shells when it doesn't have a charge.

Now fill up the shells one by one
Two in the first shell
4 in the second

That means there are four empty places in the outermost shell (the valance shell)
8 - 4 = your answer

Look up the atomic number for the other two and fill up the shells then you can easily work out how many additional electrons you need to complete each shell.

I hope this has made it clearer, have a go at the other two if you get stuck just post back