Telling you the best way to learn new vocab would be entirely opinion-based. However, I can suggest alternatives to and offer advice on the standard answer of looking in a dictionary.
A dictionary won't tell you how popular or unpopular a word is. If a non-native speaker finds a new word in a dictionary or thesaurus they might even find themselves using a word that no native speakers are familiar with! A dictionary isn't necessarily the best place to go to first to learn new words.
You would be better off giving yourself exposure to native English speakers - either through practical experience speaking with others or even watching videos of conversational English. Pick out words that are being used and look those up.
If you are having trouble understanding dictionary definitions then perhaps try a children's dictionary, as these use simplified English in their definitions. There are several online, such as Kids.Wordsmyth. Dictionaries also contain example sentences using the word or phrase, so make use of these to see how they are used in context.
You could use a bilingual dictionary to look up the word in your own language, but the problem with this approach is that the idiomatic use may be entirely different.
Remember that learning a second language involves acquiring not just a vocabulary of individual words but phrases and structure. In fact, once you begin using your second language you will find you learn new words in "chunks", just as you probably learned to say "good morning" before you learned what "good" and "morning" meant individually. Practical experience speaking and receiving any language is more useful than looking in a dictionary.