Luisa
 

It's listed on Amazon with a couple of photo's, one of which shows the handle-bar. These are Grip-Shift twist shifters that changes the gearing. The right shifter controls the rear "cassette cogs", the left shifter the front "chainrings" Those are the terms for the gearing system. You twist them to change gears.

The smaller cogs (with fewer teeth) are the harder gears, used when you are going faster. You twist the shifters to change cogs to harder or easier gears in terms of effort. As the terrain gets hillier, you move up the cassette to larger cogs, which allows easier pedaling as the hill gets steeper. Eventually the effort is too great for the steepness and you then shift the front chainring to the smallest ring. That provides for a new range of gearing in the rear. Likewise if you were going down a hill you would shift the front to the largest chainring to get you moving faster.

Best advice is take the bike out on an empty parking lot, shift away on both front and rear, pedaling while you do so till you learn how the system works.