Using gears to create mechanical advantage is undisputedly a very important advancement that is utilized in many/all feilds of engineering.
This instructable will give you a quick crash course on finding, analyzing, and utilizing gear ratios.
Step 1: Finding Gear Ratio
This step is pretty simple, just count the teeth on your two gears and identify which gear will be your input and which will be your output.
I have two different pictures here to represent the two options you could chose.
Step 2: Analyzing Gear Ratio (Speed)
Put your teeth numbers into the format output/input.
In this case it will be 18/30, or if you divide it out, .6.
This number means that the input gear has to spin .6 times for the output gear to spin once. This set up will provide you with speed, but know that if speed goes up, torque goes down, meaning that the input gear in this case will be harder to spin.
Step 3: Analyzing Gear Ratio (Torque)
In this case the ratio will be 30/18, or if you divide it out 1.66.
This number means that the input gear has to spin 1.66 times for the output gear to spin once. This set up will provide you with torque, but know that if torque goes up, speed goes down, meaning that the input gear in this case will spin slower.