Hi there Fernando, To speak respectfully in English, you change your phrasing to match a formal setting. English differs from Spanish in that there is no formal second-person pronoun (I believe it's "usted"?) However, instead of saying, "Hi, how are you?" you would say, "Hello, how are you doing?" Typically, formal/business English does not use shortened and contracted forms of words and phrases. Also, you would use titles where appropriate. For example: (Casual) Formal (Thanks!) Thank you. (No problem!) You are welcome. (I'm just going to the washroom. Be right back!) Excuse me, I need to use the facilities. (Hello, Sam!) Hello, Mr. Smith. Hope this helps!

there is a lot of slang in informal conversation

This is a huge topic. Where do you plan to start?