As Terry Waltz mentions in her book "TPRS with Chinese characteristics: making students fluent and literate through comprehensible input'': Language is a universal human ability and the human brain is hard-wired to acquire multiple languages.
Think of when you were a toddler and you watched and listened to other people from your surrounding talk. At first you would only listen and watch and then you would start to understand what your parents would say to you. You then started to speak, and read and write your own mother tongue. You do not remember it as being hard work, it just came naturally to you!
You can do exactly the same when it comes to knowing another language. Whichever you choose: French, Spanish...., your primary goal is to become fluent and proficient. In other words your goal is to be able to speak and understand the language without effort and to deal with the situations you need to deal with in that language.
Even when you already have a busy life and don't have enough time to dedicate to another language, you can do it! Instead of learning the language, acquire it the natural way in 5 simple steps:
Stay home and take online lessons! When you are in the comfort of your home, it helps lowering the affective filter. You are in a safe and affirming environment which lowers your stress and anxiety levels and encourages you take risks and view errors as a natural progression of language acquisition. You are self-confident and relaxed and therefore you will succeed at acquiring the language. The other benefits of being at home is that you can organize the way you want to acquire the language at the time you want. You save time, you focus on the language straight away and you quickly go back to your day job when the session is over.
Practice regularly for short amount of times. You know the saying: "A little goes a long way". Acquiring a new language takes a lot of brain work so it is best to practice for maximum 30 minutes in a row so that you brain can process the new structures and vocabulary.
Acquire the high frequency words. Did you know that most people only use around 700 words (if not less) in their everyday conversations? Instead of memorizing huge lists of vocabulary, aim at acquiring the most frequent words used in the language you want to know, so that you can understand 90% of the spoken language.
Make connections. It takes a lot of time to remember words from a word list whereas it seems much easier to remember a story you have heard or a movie you have seen. Why is that? Because your brain needs connections. Learning a word from a word list establishes one connection, and therefore, does not remain in your long-term memory. However, if this word is embedded in an exciting context, many more connections are generated. Because of all these connections, our brain stores the new word effortlessly. When you are acquiring another language, you should create and tell stories, have conversations and discussions which interest you.
Listen and read about what interests you. You should get a lot of input, which means you should listen to the new language a lot and you should also read in the new language. Reading is something you didn't do when you acquired your mother tongue, hence it took many years for you to acquire it. The great news about acquiring another language when you are an adult is that it doesn't take so long because you can read! Be sure to listen to and read stuff you can understand and which are of interest to you. This is how you will keep at it!
Remember that acquiring (and not learning) a language should be an enjoyable experience. When you laugh and have fun, you will make it!