The best way to differentiate between first language and second language is to think of the former as the language you learned naturally from your parent right from when you were still a baby, whereas a second language is any language you learned other than your native language. A primary language is usually regarded as a native language. You don't struggle to learn this type of language.
The learning process is very easy because you are not afraid to make mistakes. Another reason is that almost everybody in your locality is speaking the same language, so you find it easy to get yourself used to some words, sentences, and how they are used. In contrast, the learning process of any second language is a difficult one, except for some people with certain abilities.
A second language can be any language other than your native language. One of the reasons that make the learning process of a second language very difficult is because you only listen to your teacher, and in most cases, you don't have more people to speak the language to, plus the fear of making mistakes.