At the VERY basic level, (assuming it's the one
Evans are selling
here)...
3 chain rings at the front, controlled by the lever on the left of the handlebars. Smallest ring, very easy pedalling, middle ring fairly easy pedalling, big ring harder pedalling.
9 cogs on the rear, controlled by the lever on the right of the handlebar. Ranges from biggest cog with most teeth = easy pedalling to smallest cog with least teeth = harder pedalling.
In general you can use the middle ring at the front and all 9 cogs at the back (which will do for most riding) but preferably only the 7 biggest cogs with the smaller ring at the front and the 7 cogs with the smallest teeth with the bigger ring at the front. When you get the bike, just go out and find a quiet piece of road, car park or similar and play around with the different combinations to get a feel for what suits your style of riding.