The apostrophe (') is the most troublesome punctuation mark in English, and perhaps also the least useful. No other punctuation mark causes so much bewilderment, or is so often misused. On the one hand, shops offer *pizza's, *video's, *greeting's cards and *ladie's clothing; on the other, they offer *childrens shoes and *artists supplies. The confusion about apostrophes is so great, in comparison with the small amount of useful work they perform, that many distinguished writers and linguists have argued that the best way of eliminating the confusion would be to get rid of this troublesome squiggle altogether and never use it at all.
They are probably right, but unfortunately the apostrophe has not been abolished yet, and it is a blunt fact that the incorrect use of apostrophes will make your writing look illiterate more quickly than almost any other kind of mistake. I'm afraid, therefore, that, if you find apostrophes difficult, you will just have to grit your teeth and get down to work.