Here is part of a very famous speech by Winston Churchill. One of them is the way he wrote it. The other has been modified. In your opinion, which is better?

a) We shall go on to the end. We shall fight in France, we shall fight on the seas and oceans, we shall fight with growing confidence and growing strength in the air, we shall defend our island, whatever the cost may be. We shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; we shall never surrender.

a) We shall continue until there is a conclusion. We shall fight in France, we shall struggle on the seas and oceans, we shall contend with growing confidence and increasing strength in the air, we shall defend our island, whatever the cost may be. We shall engage on the beaches, we shall combat on the landing grounds, we shall battle in the fields and in the streets, we shall skirmish in the hills; we shall never surrender.

Don't try to learn them that systematically. Native speakers pick them up slowly over our lifetime, learn their meanings from context, and only go to the dictionary for a reminder, or when we realize we only know a word from context and are not quite sure what they mean.

Try to identify the most common word. If you have a short learner's dictionary, it's the one that's in your short dictionary. Use that one. When in doubt, a short one-syllable word is likely to be the main word. English actually has a sort of dual vocabulary in which many nouns have a pair of synonyms: a short one from Anglo-Saxon ("dark"), and a longer one from Latin ("obscure.") 

Use the basic word yourself. Don't deliberately try to use synonyms yourself in your own writing. It isn't necessary, and it isn't even always an improvement in style. Sometime writers use synonyms for variety, but sometimes they deliberately use the same word over and over again to create a kind of rhythm and emphasis. 

Learn the synonyms so that you will understand them. Don't try to learn the subtle differences in meaning before you need to know them. Let yourself pick them up through context as you encounter them.

There are a few cases where two "synonyms" don't really have the same meaning, and careful writers think it's important to use the right one.

"Thesaurus" is the name for a kind of reference book that is organized to help find synonyms. It's best used by writers who already have a very good command of the English language, already have an idea of the word they want to use, and are looking for a reminder. A writer might have the feeling that they want to call a dark forest "tenebrous," and want to check on whether it just means "dark" or whether it also carries the idea of "depressing." (It does). 

http://www.thesaurus.com/

https://www.collinsdictionary.com/dictionary/english-thesaurus

For sure the learning of synonym is important to expand your vocabulary capacity to a degree but do not forget learning through context and free association is also critical to retain them firmly in your mind. 

To Dan Smith:

I like the latter one.