A GOOD DICTIONARY
To learn a new language it requires a lot of vocabularies that are new to us and we need to know about their meanings. Sometimes it is good to have a monolingual dictionary, which means the English word is defined in our own language. But it is better to have an English to English dictionary because it not only gives us the meaning of our concerned word but it gives us an extra new word which increases our knowledge.
For beginners, Oxford Advanced Learner's Dictionary is the best.
If you have some knowledge of English and are passed the beginners, then Cambridge Advanced Learner's Dictionary is the best choice for you.
The good thing about Cambridge Advanced Learner’s Dictionary is that it has lots of examples for the new word you are looking up and it also has a part for common mistakes that new learners frequently make.
There is another useful part in the dictionary and that is highlighted words that are very much used in English conversation.
In general, the dictionary can be a great help in many ways to English learners.
If you are looking for a good online dictionary, then Merriam-Webster has a more intuitive interface while Oxford has marginally more words and better definitions.
Dictionary.com - The world's favorite online dictionary! is not too great (has poor definitions and sub-par example use), but it will work just fine if you’re more used to websites with many hyperlinks.
Another help for you to find your new word is Vocabulary.com. It not only gives you the meaning but also informs you about the history behind the word and how much it is used in daily conversations.
Most of the dictionaries provide plain answers. They don’t make it all clear to where and in what situations we can use a particular word but with Vocabulary.com you can have the information for the special situations too.
There may be other apps or online facilities that you may find in your place but these were just a few suggestions for you.