The biggest mistake is to be afraid of making mistakes.

I agree with Nilton. Never be afraid to make mistakes - thats what anyone experienced in learning languages would tell you. Persistance is the key and my number one advice to you would be to keep making mistakes. Just remember when making mistakes to listen to when people correct you so you don't make the same mistake time and time again. Listening to your teachers advice when making a mistake and acknowledging you made a mistake will work wonders. Never make the same mistake twice, only make it once and your journey will be much shorter. I wish you the very best with your Language learning journey, sir.

One of my mistakes was trying to read newspapers in Spanish. The grammer was too sophisticated and the vocabulary is not representative of everyday conversation.

From my point of view the biggest mistake I have ever made is learning new words by translating them into my native language at first instead of trying to understand their meaning from the context. 

Mi huge mistake is that, most all the time, since I usually read instead of practicing my English, when I learn a new word I don't pay attention to the pronoutiation and, when I try to use it I do it incorrectly, Then I have to relearn this word, linking  the correct sound and its meaning.