To participate in the Forums just log in with your profile on Domestika (if you don't have it yet, here you can see how to create a profile) and go to the Forums section, which you will find in the website top menu.
Once inside, you can submit a new topic and add it to the corresponding subject, in order to share your question, idea, or recommendation with the rest of the community.
If you are going to ask a question, we recommend that you take a look in the forums to see if it hasn't already been answered, to avoid repetitive content. We also recommend that you read this article about using the Forum before posting a new topic.