As most people already mentioned, the main thing is broker`s reliability. Before starting trading, you should make sure, that the broker is not a scammer. How to do it?
First of all, check the broker`s license. They say, that the most reliable brokers have European and US licenses. They`re considered as the most difficult to obtain and they have the most strict regulation.
On the other hand, there are brokers with licenses from Panama, Hong Kong, Mauritius, Belize, Seychelles. They have less strict regulation, but it still better, that no license at all. Also, such brokers usually have more profitable trading conditions. It`s a big advantage, but you should carefully choose such broker, read reviews. If the broker works for 5 years and more, carries about it`s reputation, don`t have many bad reviews, you can try it.
Trading terms is also a popular aspect while choosing a broker. But in this case, everything depends on you plan and strategy.
Also pay attention to withdrawal methods.