Wait for your activity to update. You can look at it tomorrow and post atleast one so your activity would be updated. You will surely be a Jr Member when your activity updates that is 28 (your current activity) + 14 (next update) = 42 activity. You might want to consider reading the pinned post and read for better understanding.
P.S. The activity updates every 2 weeks and this week it will update then you will wait again for 2 weeks particularly tuesday but surely wednesday.
This is well explained, I had the same problem before and failed to read the thread about activities and posting and about the 2 week interval for where it'll update the 14 activities. My suggestion too is that if your on your laptop or phone, you could set your calendar evey 2 weeks and check around Wednesday if your activity increased.