Help I found an acount.

Yes. Steam will ban users related to hijackin of other accounts.

What can I do? I didn't do anything. I just enter in the account. Now I change the password so anyone except me can enter. But I want to give back to the true owner. How can I say this to the steam support? They are going to believe me?
In addition, I heard that the spanish steam support is not so good than the english steam support.

Why would you even try to log into another persons account, and then proceed to change the password?

I have no idea if steam support will believe your story, but they will find the account one way or another and they will probably associate it with your IP address.

edit: find the forum post or wherever you got the account details before you entered them, bookmark that page and take a screenshot, that might be good enough proof to clear things up with steam support.

hm, okay

Next time think twice before doing something stupid. If you already got your own accound why in hell would you want to login onto other's??

Did you actually use the account other than log in to it? E.g. try to play games? If so you'll have some explaining to do to support because you will look plainly like a thief. Sounds like you didn't even know who owned the account, which doesn't work in your favour either.

Imagine you're walking along the street and you see someone pick up a phone they found on the ground. They see you walking past and offer to you, "Here, you can have this". Taking that phone and making it yours doesn't really sound like the right thing to do does it?

Como hablas español y me da flojera escribir en inglés xD, parece que lo "regaló" y luego reclamo su cuenta esperando que otra persona pusiera juegos ahí, de qué grupo lo tomaste? porque parece que este tipo intento hacer alguna estafa de algún tipo con su dizque sorteo

You can't sell or gift steam accounts, it's forbbiden by stream TOS.

Si lees los Acuerdos de Suscriptor que aceptaste al crear tu cuenta, pone "No puede vender ni transferir a terceros el derecho a utilizar su cuenta ni cobrar nada por utilizarla". No vendas o regales tu cuenta ni aceptes la de otro, ya que solo te puede traer problemas. Haces muy bien en devolverla, pero (como te aconsejan más arriba) haz una captura y reúne todas las pruebas que puedas por si te acusasen de ser tú el que robó la cuenta. Suerte. :-)

Wow, such trap, very tricky.

and that's why i dont use facebook

I'll give you the gun that I used to kill my parents last week. It's a giveaway so I don't see any reason why you wouldn't want to accept it...

Are you that dumb? After 5 years of being on Steam, one would expect that you already know that gifting \ reselling a Steam account is forbidden. And we're still wondering why there are more and more scams on Steam, the reason is there are still stupid enough people who fall for these 'offers'...