The broker must be authorized to provide investment services in your country and provide you with its licence details. You shall be able to verify the licence with your national regulator. In Cyprus, for example, the market is regulated by CySEC and you can verify your broker with the regulator’s lists.
The broker should provide services in your language and should be able to communicate in the way you prefer (in person, by phone, email or chat). Check its competence not only on the broker’s website, but you can also use an independent comparison websites and reviewers, however be aware of the fact that they may also contain advertisement. Examples: https://forbino.com/en/broker-comparison/, Trader-magazine.com
The broker should provide you, among others, with the following information in advance: Risk disclosure, Trading terms and conditions, Privacy rules. An overview of the legal framework can be found in educational articles, e.g. on Investopedia (https://www.investopedia.com/terms/m/mifid.asp).