to reduice variance you need to spend points in that line, I'm going full mana at this moments, should I stay like this until minidings or should I start adding some points from low level to have two good DoTs?

Depends on your level, your main goals (pve / rvr) and your playstyle (solo, smallman, fullgroup)

For pve, I would always keep Mana on level (gains so much for every playstyle, i.e. hot, pom, aedot) and put everything that remains after minis into mentalism.
Edit: standard specc for mana would be 46 mana, 28 menta 4 light...

In group you wont hardly ever need the second dot and solo, you can do well with either specc because mobs wont live long enough for the mentalism dot to shine.

Also if you have good reactions / reflexes, you could consider stacking endu potions (endu over time) and try around with solo or duo ae pulls (similarly to cave shaman / cabby)

The only exeption would be to invest some early points into mentalism to get the mezz for bgs. For that, you could plan your menta ahead and go i.e. one point below level into manaspecc and use the rest for mentalism.

Just play around with the charplanner a bit http://play.nixian.eu/index.php?page=ch ... =mentalist

thx for the info, maybe I will try the option of 1 under level in mana and rest into menta because I want to try BGs, is my 1st menta and need to learn the class and always thought that the only way to know a class is play it solo and grped both in PvE and RvR.