Vicky I can give three answers to this:
First answer is you buy when you think the price of an instrument will rise, and sell when you think the price will fall.
Second answer is you sell when the price has reached a credible resistance level and buy when the price has reached a credible support level (buy the dip). See attached screenshot.
Third answer is you can buy after price has broken through a resistance level and you need to confirm this using various methods such as looking out for a confirmation candle. You could also sell when price has fallen through a credible support level; this is known as the breakout strategy.