All trading platforms are supplied with buying and selling buttons. Those buttons are typically displayed at the upper left corner of each chart. Once you determine that you want to open a position, you simply click on the buy or sell button and a dialog window will probably appear. This window will let you set the parameters of your trade, such as the stop loss level, the target profit level, and the lot size. Once you set all or some of those details, you can click on the buy or sell button in the dialog window to execute the order.
Often, your order will be executed at the current market price plus or minus the spread offered to you by the broker. In other words, if the market price for EURUSD is 1.10080, and the broker’s spread is 1.3 pips, you will be able to buy it at 1.10093 (which is 1.10080 plus the 1.3 pips spread). If there is slippage, the price at which the order is filled will vary from the market price. You should generally refrain from dealing with brokers who do not offer good execution of trades and are associated with high slippage, as this will increase your trading costs.