There are few things that you must consider. The most important is the regulation of the broker, make sure that the broker that you are about to choose is regulated with minimum 1 Top Tier regulator like FCA, ASIC, CySEC or FSCA.
Other important thing is to check the reviews of a broker, as this will give you idea about the broker’s history of dealing with their clients, their fees, support, platforms, execution. A good broker will generally have more positive reviews by genuine clients, and also see how the broker responds to negative reviews as this is a good indication of their support.
If you are a professional trader then you should consider an ECN broker or STP broker that offers Raw spread & the lowest commissions per lot for the instruments that you mostly trade.