Charging a 3.7 V with a 5V usb charger
Ok, so you aren't actually asking the right question.  I'm going to assume you mean you have a 5V power source (usb, battery pack, etc.) and want to charge a single cell lithium ion battery with nominal voltage of 3.6V or 3.7V.  

Lithium Ion, also sometimes called Li-ion or Lithium polymer or Lipo) have a slightly different charger than other battery types.  Typically, the charger supplies a constant current at slightly higher than the battery's voltage.  As the battery voltage increases, the supplied voltage increases until it hits the maximum for the cell - typically 4.2V.  Then, the voltage is kept constant and the current is reduced until it hits 3-5% and charging is considered done. 

All of that is to say, that you need a specific charging circuit and an IC dedicated to charging a single cell lipo.  You could build one, but that seems a little advanced for you at the moment.  If you decide to, I detail my experiences here: https://thewanderingengineer.com/2016/01/27/simple-lipo-battery-charger-with-the-mcp73831/

A better option might be to buy a board that does this for you so that you just plug in a usb plug on one end and a single cell lipo on the other end and you don't have to worry about it.  Here are links to a couple of products from sites that also have good tutorials about them: 
https://www.sparkfun.com/products/10401
https://www.sparkfun.com/products/12711
https://www.adafruit.com/products/259