You should start by choosing the right forum - if you want to ask a specific question, related to a particular game available on GOG.com, you should post in its game-specific forum.
For any other questions, just use the General forum.
When posting, consider selecting the "Mark as question / problem" option - this will highlight your thread, and allow you to mark one of the posts as the solution. This will help other users, if they're looking for an answer to the same question.
Please keep in mind that the GOG support staff does not frequent the forum and cannot usually help you if you post in the forums. If you would like to contact our support directly or if people from the forum are unable to help you solve your issue, please open a ticket.