You have a number of options:
You can close the trade, and take the loss
[ii] You can hold the trade, and hope
[iii] You can trade your way out of it
If you are a 'novice' then closing the trade and taking the loss is probably the best way to go. That you are even asking the question rather identifies you in this category. That is not a criticism, we've all been there.
Closing your eyes and hoping is not the way to go.
Trading your way out can be the best solution but it depends on your leverage, your available trading capital and your timeframe.
If you were to attempt this way then there are a number of possible strategies:
Trade it based on the technicals. On the 1 year chart, it is approaching support. My prices are US based. I have current price at $68.43 Support is at $65.oo'ish. You add to your position. Then calculate a stop-loss, a break-even point, and where you start generating profit. You close the entire position at a hard stop. You close the new position at B/E and close the remainder at your original profit target. Obviously your risk has changed quite significantly if you choose this option.
[ii] Convert the position into a pairs trade, thus converting the position in theory to a market neutral trade. The profit is earned on closing the spread. RIO would be the 'usual' pair, but, the trade is already upside down. A good possibility is the US ETF XLB. There is currently a 15% spread available. So in this trade you add to the BHP position and sell short XHB to finance the trade.
[iii] Add an Option position to the trade. Essentially hedge the position with a PUT position. I would favour a 'Backspread' position. The difficulty is in obtaining a credit. However, you can make it work with a low debit on the position as of course you hold the stock long. I would recommend about 6mths on the trade.
jog on
duc