No, you don't get taxed on your capital, just as you don't get taxed on your money sitting in a bank (which is also capital). If your broker pays interest on that capital, then like bank interest, you will pay tax on the broker interest received. If the capital is used to buy shares, then you will pay tax on any dividend received or any capital gains made. If you make a capital loss, you will be able to write that loss off against other capital gains (how capital gains tax works is a bit more complex that the way I have just stated it here).
Your capital can increase by making more deposits or by selling assets at a greater price than you bought them at (you pay tax on the gain only). It can decrease if you withdraw funds or if you sell shares for less than you paid for them (in the latter case you will have a capital loss that you can write off against other capital gains).
There are no tax implications for the actual withdrawal of funds. There are only tax implications if there is interest or dividends paid, or a capital gain/loss.
