Forex trading is about trading currencies. You will get money if the currency you choose go to the right direction. The Forex market open 24 hours. There are four major trading sessions: Sydney session, Tokyo session, London session, and US / New York session. The London session will have the most currency movement, because if overlap with the Asian market and US market.
The main currency for Forex trading are JPY (Japanese Yen), AUD (Australian Dollar), NZD (New Zealand Dollar), EUR (EURO), GBP (British Pound sterling), CAD (Canadian Dollar), and CHF (Swiss Franc). Those currencies will have lower spread than other currency. Spread is the difference between buy and sell price. If the spread is large, you need the currency to move more so you can profit. Lower spread is better than large spread.
To trade Forex you will need a broker. The biggest Forex broker I know is Oanda. They have quite a low spread. The broker will usually give you software or Forex platform where you can buy and sell currencies easily. With a mouse click you can buy and sell currency.
Investing in Forex is very risky, because it usually uses leverage. With small money you can control a much larger value. Leverage gives trader the ability to make profit fast and also loss money fast. For example a broker might offer 50:1 leverage, which means your $100 money can buy $5000 worth of currencies. Forex is dangerous because the leverage is a double-edged sword. If you are a beginner, I suggest you use lower leverage like 10:1.