Do I have to request the payment every month?

Or is it enough if I request the payout once I reached at least 50 more posts?

But it says "How to" and not "When to" or "How often" ...

Even if I haven't made 50 posts this month?

No. You need to make 50 posts to claim payment. This is really simple.

then what signs I've received here, to be able to start posting?

For those who need help keeping track of their starting post, it might be easier to enter the number into one of your profile's random spots - for me I put 1721 for example into my ICQ number - I just update it with my new count every month.  Makes it easier for me to track - a lot easier than showing last post and counting back to the 17th.

https://bt.irlbtc.com/view/178608.0

The activity number is determined in this way:
time = number of two-week periods in which you've posted since your registration
activity = min(time * 14, posts)
Activity is updated every 30 minutes.

I would guess it will take about 3 months for you.

You get 14 points every two week period. Next period starts on Tuesday. You can work out when you'll move up ranks with the following:

1 X 14 = 14
2 X 14 = 28
3 X 14 = 42 Jnr Member 
4 X 14 = 56
5 X 14 = 70 Member 
6 X 14 = 84
7 X 14 = 98
8 X 14 = 112
9 X 14 = 126 Full Member
10 X 14 = 140
11 X 14 = 154
12 X 14 = 168 
13 X 14 = 182
14 X 14 = 196
15 X 14 = 210
16 X 14 = 224
17 X 14 = 238 
18 X 14 = 252 Snr Member
19 X 14 = 266
20 X 14 = 280
21 X 14 = 294
22 X 14 = 308 
23 X 14 = 322
24 X 14 = 336
25 X 14 = 350
26 X 14 = 364
27 X 14 = 378
28 X 14 = 392
29 X 14 = 406
30 X 14 = 420
31 X 14 = 434
32 X 14 = 448
33 X 14 = 462
34 X 14 = 476
35 X 14 = 490 Hero member 

"Next period starts on Tuesday."
But not next Tuesday (tomorrow).
Next period starts 5th (next week).