What an indicator is
An indicator is a calculation drawn on top of your chart to help you read price action — a moving average, for instance, smooths the price into a line so a trend is easier to see. MetaTrader ships with dozens of built-in indicators, and you can add downloaded ones too.
Indicators do not predict the future; they summarise the past in a particular way. Used sparingly they can clarify what you are looking at. Piled on top of each other they mostly create noise and false confidence.
The numbered steps
Step 1: Open the Navigator panel (Ctrl+N) and expand the "Indicators" list, or use the top menu Insert → Indicators.
Step 2: Find the indicator you want — for example "Moving Average" under Trend. Double-click it, or drag it onto your chart.
Step 3: A settings window opens. Adjust the parameters (for a moving average, the period and the method) and the colour, then click OK.
Step 4: The indicator appears on your chart. Some sit on the price itself (like moving averages); others open in a separate panel below (like the RSI or MACD).
Editing and removing
To change an indicator later, right-click on it (or right-click the chart and choose "Indicators List"), select it, and open its properties. To remove one, right-click it and choose "Delete Indicator", or delete it from the Indicators List window.
On MT5 the menus look slightly different but the ideas are identical: Navigator to add, right-click to edit or remove.
A caution against clutter
It is tempting to add five indicators at once, but beginners who do this usually end up more confused, not less. Each indicator is just a re-arrangement of the same price data, so several of them often say the same thing in different colours.
Start with one, learn what it actually tells you, and only add another if it answers a different question. A clean chart with one well-understood tool beats a crowded one you cannot read.
An honest note
No indicator, and no combination of indicators, is a reliable buy or sell signal on its own. They are aids to reading the market, not a system that removes the risk of loss. Be especially wary of any downloaded indicator sold as a guaranteed money-maker.