Count the number of sprockets you have up front, then count the number you have in the rear, Multiply, 3 x 6 = 18 speeds. 3 up front, 6 in the rear, or 2 up front, 5 in the rear, 10 speed. Etc, they go as high as 21. You might only have one in the front.
To shift, you have to be peddling, get the bike going, put the chain on the middle or the biggest sprocket in the front, move the left thumb shifter to do this, do not let the chain rattle, make it a clean shift and don't let it klik or rub. Then put the chain on the biggest one in the rear , without any rubbing, chattering, slipping, etc,with the right shifter, now you are ready to take off easily, when you get going and the pedals get too easy and go faster, shift the rear to the next sprocket down, then again, then when you are coming to a stop, down shift the rear sprocket back to the biggest gear and you will be ready to take off again easily. A few tries, you will shifting like mario andretti. Hope this helps.