I understad how primary stats (e.g. aim) works, but I am still lost concerning secondary stats (e.g. power, critical rating, accuracy, etc.) I am not sure if they are very important and if I should pay close attention to them or if they are only something I should bother with when I am 50+ level... And I don't have a clue which secondary stat I should give a priority. What it depends on? Roles, like if my character is DPS, Tank or Healer? Or if he uses meele or ranged weapons? Or it depends on advanced class?

And should I concetrate on one secondary stat or more?

Any help would be appreciated.

This might be a helpful link to your question. http://www.swtorhub.com/advanced-stats-guide

Not sure if that is as up to date for more recent patches, but it explains what is what and why. Even a nifty chart near the bottom, broken down for each advanced class, and role they are.

Yeah they depend on your role.

If DPS: power = it adds damage to your abilities. Some DPS have crit-based talents that give them extra damage if they crit, so in that case crit may be an option instead of power, or in addition to power. As DPS you also may have to worry about a little extra accuracy so that your hits land on boss-level enemies. But mostly power.

If tank: you have to get a balance of Defense, Shield, and Absorb. Defense reduces damage (as if you had extra armor on you), Shield makes the shield function more often, and Absorb makes the shield negate more damage.

If healer: power (so you heal for more), and you don't need accuracy because your heals always hit your friends, so maybe some alacrity (haste) so that your long heals are shorter to cast.

It really only matters at max level, for some of the raid content. Leveling up, don't worry about it too much, just get whatever's available on the GTN for cheap.

Just a brief correction: Defense increases your chance to avoid entirely Melee/Ranged attacks ("white damage").

But yeah:

DPS wants enough accuracy to get 100% Melee or Ranged, and then a mix of Power, Surge, and Crit (possibly also some Alacrity depending on what your class is).

Healers want Power, Surge, Crit, and Alacrity.

Tanks want Shield, Absorb, and Defense exclusively.

I recommend checking out the SWTOR Class Guides on Dulfy.net; they give breakdowns of what stats you want to pick up based on what your class and spec are. Before you hit 55, though, pretty much anything goes; don't worry about the minutiae of stat distribution until then, so long as you're wearing the right mainstat.