Well, is it for a living or for a hobby? It makes a difference. If you are trading for a living, your gains are treated as regular income and you pay your regular tax rate. All of your trading expenses are deductible. If you have a loss, you can deduct it against any salaried income you have, you might even be able to claim back against income from previous years, but I'm not sure on that.
If it's a 'hobby' or just regular ol' investing, then stock trades are treated as a capital gain or capital loss. Taxes are calculated by taking 50% of your profit and paying your normal rate on that amount. Losses are not deductible against anything except other capital gains, but they can be carried forward, and claimed back against capital gains from prior years. You can't deduct trading expenses, but you can deduct the full amount of interest you paid, such as an investment loan or margin interest.
BTW, if you are short selling or doing any derivitive stuff, your gains will be treated as regular income, since the government doesn't consider this 'investing'. I think some kinds of option trades can be accepted as capital gains, not 100% sure on the rules there.