For forex trading, most traders prefer using the MT4 because it is easy to use and it is mostly used for forex, but after the release of the advanced version of the MT4 many traders also prefer the MT5 because it can be used to trade stocks as well.
I agree with @Bradley, MT4 is good to start with. I am using it for more than a year now with turnkeyforex. Low spreads and commissions, good client support and easy to use trading tools for beginners. You can practice on their demo account until you become confident with real market conditions.