I'm really a no-no when talking about singing... I really suck, I'm the worst singer ever.
Is there techniques or can I learn to sing? IS this a "learnable" thing? lol!

Portuguese is such a sweet and lyrical tongue, I can't imagine how anyone could sound bad singing it. Although I don't speak or understand a lick of it myself, been listening to Jobim with Elis Regina Carvalho Costa lately... if you practice by singing along with them, you can't possibly go wrong.

Howard

One of the best ways to improve on singing is to join a choir. Like anything in music it will improve if you do enough of it. Just listen carefully and practice little and often things will improve for you.
Cheers

I would take some lessons and get a coach. Maybe try singing some different type of music to expand and see what type of singer you are and learn your strengths and weaknesses.

I know what you are talking about, I don't even dare to sing when I'm all by myself

Anyone can learn to sing. Yes there are techniques. There is a reason that singers train for years. Some people do have natural aptitude and others work hard for years, but both can end up with the same results (or more likely, the hard worker will get better results). You are only limited by your own perception of yourself.

You don't say exactly what your issues are, but my advice is this:
Find a good teacher. Learn how to breathe to support your voice. That will get you started.