It's been a while since I became one but if I remember correctly, you need to have posted on the forums at least 50 times over a period of time (you can't just post 50 posts in one day and expect to get approved) and then you have to file an application to become one for the staff to check and after they've checked it out and made sure that you fit the bill, you'll be approved.
Hi there! As kibou97 mentioned, in order to apply for full membership you will need to have a minimum of 50 posts, and been a member for at least two weeks. The 50 posts should be made in different areas of the community, which will allow us to evaluate your application better. Please do not make 23 or 24 posts in one day so to be able to submit the application. Once you meet the basic criteria, you can complete and submit an application. If you have any questions please do not hesitate to let us know.