I did not change any meter modes as I don't even know how.
What did I do wrong?
What you have done wrong is you apparently haven't yet read the D3100 User's Manual so you know what basic exposure adjustments are available to you.
Read the manual with your camera at hand so you can make the changes the manual describes. In fact, you will need to read the manual several times, and always keep it handy for reference in your gear bag.
See pages 79-32, that's the user's manual section that covers the D3100's metering features. They explain the 3 metering modes you have available to choose from, exposure compensation, and exposure lock.
It's like you're trying to learn how to drive a car, without first finding out how to change from parking lights, to the regular headlights, to the high beams.