To run a test, you must first import or load a data file, then open or create a script that includes one or more Strategy sections.
If the currently active script includes a Strategy section, then the Test button on the Tool Bar (or the Test item on the Run Menu) will be enabled. Pressing it will run the script in backtest mode.
As with all run modes, Test first applies your settings from the Settings Panel, then applies the Settings script section if one is present. Finally, if your script includes a Test Settings section, items that it specifies will override your general-purpose settings for those items.
If you haven't already done so, please go through Tutorial 1 and perhaps the others to get a feel for how backtesting works in RealTest.
For a look "under the hood", see Backtest Engine Details.