Hi! I'm a native English speaker who has learned Vietnamese, and is currently learning Spanish. The best thing to do is take a class, maybe online or locally. It helps to learn the basics, like pronouns, greetings, goodbyes, and simple conjugations. It varies for different languages. For example, in Vietnamese, you would need to learn different accents and how to differentiate how those would affect the pronunciation.
However, in Spanish, you would focus on present conjunctions, and the masculinity and femininity of certain words. Then, you would gradually add more and more knowledge to your database and learn how to create more complex sentences.
I totally definitely recommend using Duolingo to learn and practice different languages. Rosetta Stone is also another great resource.