I think to some extent you may be confusing the terms margin and leverage.
Two concepts that are important to traders are margin and leverage. Margin is a loan extended by your broker that allows you to leverage the funds and securities in your account to enter larger trades. In order to use margin, you must open and be approved for a margin account. The loan is collateralized by the securities and cash in your margin account. The borrowed money doesn't come free, however; it has to be paid back with interest. If you are a day trader or scalper this may not be a concern; but if you are a swing trader, you can expect to pay between 5 and 10% interest on the borrowed money, or margin.
Going hand-in-hand with margin is leverage; you use margin to create leverage. Leverage is the increased buying power that is available to margin account holders. Essentially, leverage allows you to pay less than full price for a trade, giving you the ability to enter larger positions than would be possible with your account funds alone. Leverage is expressed as a ratio. A 2:1 leverage, for example, means that you would be able to hold a position that is twice the value of your trading account. If you had $25,000 in your trading account with 2:1 leverage, you would be able to purchase $50,000 worth of stock.
Margin refers to essentially buying with borrowed money. This must be paid back, with interest. You also may have a "margin call" forcing you to liquidate assets if you go beyond your margin limits.
Leverage can be achieved in a number of ways when investing, one of which is investing with a margin account.