Quite a complicated question, because the broker that suits one person may be unsuitable for the other one. So, actually, you should talk to yourself, first of all, and answer such questions:
- what is my strategy (because if you’d like to scalp, you should find the broker that allows it)
- how much money do I want to trade (there are brokers with no minimal deposits on some accounts, for example)
- how trustworthy the broker is (read reviews from different sources; you may also read comments and ask people on forums about particular broker, but you should filter the info as people lie sometimes)
- how quick, polite and helpful the broker’s support is (try online support)
If everything is OK, then you may open Demo account and trade on it until you are sure, that it’s your broker!