Welcome to the forum!
So, essentially posting on the forum is rather simple. Just navigate the forums on the left-hand pane and respond to threads with either knowledge or questions.
Here is a list of our forums rules, I'd suggest perusing them to ensure you don't break any: https://www.blackhatworld.com/help/terms/
PS: the rule in question that I'm referencing is
3.5 Engaging in post count inflation by posting excessive amounts of short replies for the sole purpose of inflating your post count. This includes the infamous “thanks for the share.” (If you’d like to say “thanks” to someone for their effort/contribution, make use of the Like button).
Also, here is a link to the member levels & permissions page: https://www.blackhatworld.com/seo/updated-bhw-member-levels.889577/
To buy Jr. VIP (and afterwards a Marketplace thread for $30), you need 100 posts (messages) and 60 days on the forum.
You'll get there in no time, just don't break the rules, and you're
golden!
Good luck & have fun!