When you start language, you will probably buy a textbook, starting from Lesson 1. I will say a textbook is a good idea, but you do not need to try your best to memorize every single word in the textbook.
Instead, you should be conscious on the two things:
- how many words you need
- what kind of words you need
The vocabularies you should learn first consist of the following two components;
- the basic words
- the words you need according to your own situation
The first category above can again be segmented into the two different types as;
- Words that are frequently used
- words that appear in a variety of contexts
A textbook might cover most of the basic words, but are not relevant to your own needs. Customizing your vocabulary to suit your demand will lead you to success.
The basic words
Some words are more often used than the others. The corpus linguistics, one branch of the linguistics, tells what those words are. According to the Oxford Dictionary (see this link: http://www.oxforddictionaries.com/words/the-oec-facts-about-the-language), it tells us that ten words in the English language occupy 25% of all sentences in English. The ten words are:
Lists of corpora are available online like:
The native speakers of the language you are learning use these basic words very often, so, arguably, you need to use them as often as the native speakers do.
So, why not start learning those words first?