when a stock splits it credits shareholders of record with additional shares which are reduced in price in a comparable manner for instance in a typical 2 1 stock split if you owned 100 shares that were trading at 50 just before the split you would then own 200 shares at 25 each your broker would handle this automatically so there is nothing you need to do 
will a stock split affect my taxes no the receipt of the additional shares will not result in taxable income under existing u s law the tax basis of each share owned after the stock split will be half of what it was before the split 4