They have both + and - ve out.
Most are SMPS/regulated now, but it pays to make sure as you do not want the older style linear supply.
Max.

 

Just buy a 9V power supply (ac/dc adapter, wall wart)
Like this..http://www.amazon.com/9V-DC-Power-Adapter-Supply/dp/B004LX146Q

The power supply in the link's picture have only one cable and one output.
where is the minus output?

sorry, but i'm starting now my third year in college and i'm feeling like my questions are very stupid. how can it be?

 

There is a inner female socket usually the +ve, the outer shell visible is the -ve. as a rule.

Max.

 

sorry, but i'm starting now my third year in college and i'm feeling like my questions are very stupid. how can it be?

Because like most students you may have the book knowledge of whats taught in class but NO real world hands on skills (aka no experience)..
Most 4 year schools are severely lacking in hands on practice and always have been..

Reminds me of this video.. MIT graduates that can't light a simple light bulb