[Mutants & Masterminds 2ed]How does damage works?

[Hikarizu]

And Power vs Saving throws(Fort, Ref and Wis). Let's say we have attacker with Attack 6, "Blast 6" and "Mind Blast 6" and defender with Defence 6, Will Save 8, and Toughness 6. What happens when the attacker uses Blast/Mind Blast, when it hits, and what happens if it did?

If I understands correctly:
Blast vs Defence
Attack+d20 > Defence+10+size(if any) (This is a hit.)

Mind Blast vs Will Save
Mind Blast rank > Will Save (This is a "hit" too.)

And I am completely lost on damage though.

[The Dark Fiddler]

With damage in M&M, it's just an extra saving throw. The DC for Toughness saves (almost universally what you make for damage, but there may be a corner exception or two) is typically 15+rank, so 21 for your examples. Whoever was hit by them would have to make a Toughness save against that DC; if they succeed, they take no damage, but failure means they're bruised, and possibly more (page 163 of the rulebook has the table).

[prufock]

Toughness saves are 15+rank, so rank 6 blast or strike is Toughness DC 21. All other saves are 10+rank.

[Sir Enigma]

Toughness saves are 15+rank, so rank 6 blast or strike is Toughness DC 21. All other saves are 10+rank.

Not quite - any damage effect has a save DC of 15+damage, even if the save type has been shifted to Fortitude, Reflex, or Will. This is why the "Alternate Save" extra is +1 if you're changing a power's save from Toughness to any other, but is +0 for any other change. Nondamaging powers are the ones that have a save of 10+rank.

For the two examples in the OP:

1) Blast has two steps - attack roll and Toughness save.

-If the attack roll is higher than 10+Defense (16 in this case), the attack hits - the attacker in your example needs to roll a 10 (+6 from attack) to hit.
-On a hit, the defender must make a Toughness save of 15+attack bonus (21 for the given example). If you fail the Toughness save, there's a table that gives the result, depending on how much you missed by. Your defender needs to roll a 15 (+6 from Toughness) to successfully save.

2) Mental Blast has range Perception, so there is no attack roll. It's a damage power that's had the save shifted to Will, so the defender must make a Will saving throw with DC 15+damage bonus (21 in this case, just like the Blast). Your defender needs to roll 13 (+8 from Will save) to successfully save. If you fail, consult the same table as for the Toughness save.

[prufock]

Ah yes, good catch.