Learn to use colons and semi-colons properly.

And if you think that's easy, you are probably not using them correctly yourself. I know that my father, English graduate and author though he is, isn't.

But don't bother with colons and semi-colons unless you have the hang of apostrophes and commas. Colons and semi-colons are just the icing on the cake, but if you aren't using apostrophes and commas properly, then you aren't writing English.