Interest charged (or paid) is not related to leverage or margin.
Interest is charged AND paid on a position which remains open at your broker's roll-over time each day, and it is calculated on the value of your position, regardless of leverage.
Example: you buy one mini-lot of the EUR/USD. As you know by now, when you make this transaction, you are buying 10,000 euro and you are selling the dollar-equivalent of those 10,000 euro. Let's say that your broker rolls open positions over (into the next trading day) at 5pm every day. And let's say that at 5 pm today the price of the EUR/USD is 1.4600. That means that 1 euro = $1.46, and your 1-mini-lot position is worth 10,000 euro or $14,600.
Your broker will pay you interest on 10,000 euro at an interest rate determined by prevailing rates in the E.U. And he will charge you interest on $14,600 at an interest rate determined by prevailing rates in the U.S. So, every day, at the roll-over time, you will be paid interest, and you will be charged interest.
Whether this results in a net credit or a net debit to your account depends on the interest rates in the countries involved. For some pairs, you may find that the interest paid exactly offsets the interest charged. For other pairs, you might be paid interest if you hold a LONG position past the roll-over time, and be charged interest if you hold a SHORT position past the roll-over time --- or vice versa.
As for the amount you should use to fund your first account, you should start off as small as possible, and trade tiny positions. Find a broker who will let you open an account with $250, or even $100, and then trade one micro-lot, or less, per trade. When you have learned how to make money, rather than lose money, in this tiny starter account, then you will be ready to consider funding a larger account.
I hope that I've answered your questions.