There are lots of ways to find out what an abbreviation can mean. Dictionaries will include the most common abbreviations, but if the abbreviation is quite modern, try abbreviations.com.
If you are looking for more specialized abbreviations, there are sites for those as well. Legal abbreviations can be found at lib.law.washington.edu, medical abbreviations at medicinenet.com, texting abbreviations at txtdrop.com/abbreviations and the abbreviations for the states of the USA at 50states.com/abbreviations.
If you are learning English, there are lots of tutorials on YouTube to help you out, with some dealing specifically with abbreviations, such as this one:
