Here are some online resources you might find helpful.
1. Two Dictionaries for you.
Dictionary for English Language Learners
This Merriam-Webster dictionary is special for English language learners.
Online Dictionary with Pronunciation
This Merriam-Webster dictionary is also easy to use.
2. Collocation Dictionary from Oxford
This dictionary will show you many ways to use one word.
3. Snappy Words
This is visual dictionary that will show you how one word is associated with other words.
4. Academic Word List
This is a list of words (sorted by group) that are used frequently in university material. It is very important to know these words if you want to study in an English speaking country.
5. The Most Common English Words
Here is a list of the most common words in the English language. As you can see, they are not difficult words.
1. In the Loop
A free online book of American idioms.
2. 20 Essential American Idioms
A list of common idioms in the USA
3. Idioms and their meanings with examples
Please note that this website explains some expressions with swear words (profanity). While it is a good idea to understand the meaning of these expressions you might hear, I do not recommend that international students (or any students, for that matter) engage in swearing. I've seen students overdo it when it comes to swearing, and that can get them into trouble. Also remember, swearing usually make a bad impression (among your professors and classmates, in business situations, etc.). The best practice is to avoid using that kind of language.