Margin longing/buying of position would require you to borrow quote currency to buy base currency on the order book. Margin shorting/selling of the position would require you to borrow base currency to sell on the order book.
For a quick reference, you will see your borrowed funding amount reflected in your position's "Funding Cost" on the Trading page. Please note that if you have multiple positions, this figure may not be completely accurate on a per positions-basis, but is accurate in total.
You can also adjust your preferences on the Account - Settings page.
If you borrowed USD to fund the position, Funding Cost will accrue in USD. If you borrowed ETH to fund the position, Funding Cost will accrue in ETH.