Start by participating in the forum rather than spamming bounty threads with "Nice project". You will have to rank up to Member to allow a link in your signature before it has any value to advertisers. To do that you will need to earn merit. Read the OP of this thread to learn more https://bt.irlbtc.com/view/2818350.0

Good to know guys !

The only way to have new sMerit is by receiving them from other people for your high quality posts, only Merit sources (limited amount of people) have a specific amount that will reset each 30 days.

Good rep, yeah indirectly. If someone recieved too much merit then he for sure made good and useful post(s).
The only thing that Merit points could be used for is to rank up in this forum.

Can you lose activity over time?

Once your post(s) for a selected period(s) are deleted then you will lose activity.
For example, someone having 42 Activity (junior member) and 42 posts. He made only 1 post in his first period then this post is deleted by a mod, at final he will have 41 posts and 28 activity (he will become a newbie again).
Conclusion : make many constructive posts per period in non useless topics.

Everyone knows what post is, and you can do it how frequently you want to.

So your main question seems to be how does activity level get counted in relationship to posts.

You can earn a maximum of 14 activity points per each 2 week activity period based on your post number, and you have to have at least one post per activity period to earn up to 14 activity points per activity period.  If you average at least 14 posts per activity period, then you will earn the maximum 14 activity points per each 14 day activity period.

The OP of this thread already has some further description of activity level, and below is a link to a whole other thread discussing activity level calculations.

https://bt.irlbtc.com/view/582736.0

InvoKing is referring to a situation in which you only have 1 post for an activity period, so if that ONLY post for that activity period were deleted, then you lose all the points for that activity period. 

It's not a question of fit or unfit. Moderators will delete posts that break the forum rules in some way. Read the rules here https://bt.irlbtc.com/view/703657.0 for more details but in general, the rules most often causing posts to get deleted are.

Rule 1. No zero or low value, pointless or uninteresting posts or threads.
Rule 2. No off-topic posts.