Launch MT4 and open an account if you need to (live or demo) then you should be seeing live market data.

If not, click on Charts and then Refresh and that should fire it up.

When you are ready to trade you must open the Order window by clicking New Order in the toolbar.

Alternatively, you can right-click on a currency pair in the Market Watch and choose New Trade or right-click on an active chart and choose Trade then New Order.