There are a lot of website that offers demo trading just search it on google. First keep it on your mind that trading is a risky environment, you need patience when it comes to selling. Okay you just need buy low value currency and sell it when the value is high. For example usd to btc. Let just say 1 btc is equal to $700 now then you buy $1400 worth of btc then you have 2btc now. You need to wait for how many days or months or when you feel it is worthy to sell your bitcoin or you have a profit on it. Then when 1btc is equal to 800 then thats the time you sell your 2btc. You have a profit of $200 coz you buy it from 1btc = $700 and you sell it for $800. So you have a profit of $200. Sorry for my english.