pressing the little lever will click it to a smaller cog. So you are in the smallest at the front, and smallest at the back? Pushing the brake lever inwards will move it to a larger cog. For gears with more resistance, you will need the front Chainring in the large cog, and be using the smaller cogs at the rear. If you bought it at a shop, take it back and make them teach you, but if you bought it elsewhere go and ask them nicely at your local store
Hope this helps
Brendon