The first thing that I look are the spreads and then regulation. But even regulated brokers can scam you so this isn't a measure really.. And if the broker offers a nice deposit bonus with good conditions thats even better
I prefer broker which offers great range of currency pair including major currency pair to trade, forex demo account, real account with free bonus (as most of the brokers offer), easy Withdraw policy, mini account or macro account for starting with low investment. These are my some points that I look for in brokers.