Why do buy and sell prices differ, and why am I slightly down right after opening?

Every instrument has two prices: the ask (what you buy at) and the slightly lower bid (what you sell at). The gap between them is the spread, a normal trading cost. A buy position is valued against the bid and a sell against the ask, so a freshly opened position typically shows a small floating loss equal to the spread — the market has to move in your favour by at least the spread before you're at break-even. On top of that, a round-turn commission is charged, and positions held overnight accrue a swap. None of these are errors; they're the standard costs of trading, and you can see the spread and commission for any symbol before you place the order.