All posts above are pretty much correct.
For newbies, to start with you want to trade in a calmer, slower market period so trading during the Asian session or the middle of the US or European sessions will generally fit this condition.
Trading the US European overlap is definitely the most volatile and some experience is needed due to the speed of price movement during this period. As stated above it is possibly the most profitable if done correctly.
Market opens are very volatile as well. London open, US open, Tokyo and Singapore opens can all be periods of market flux and maybe should be avoided by newbie traders. Find information of Market opening times across the globe so you are aware of them.
Mondays and Fridays are slower than mid week, so trading from Tuesday to Thursday can be more rewarding.
Market News releases are the most dangerous times to trade and should be avoided at all costs....These events would account for the most losses in Forex so below is a link to a news release calendar...FREE
Set your timezone and try not to trade the currencies which have a red marking at the time of the news release as price can change dramatically during these periods... Please read about News Releases and their effect on the markets as "Not the best time to start"