You need to open command prompt as an administrator in a folder where your Mongo is installed (in my case: C:\Program Files\MongoDB\Server\3.4\bin). If you want to dump your whole database, you can just use:
mongodump --db database_name
You also have posibilities to dump only certain collection(s), or to dump all but certain collection(s).
If you want to dump only one collection (for example users):
mongodump --db database_name --collection users
If you want to dump all but users collection:
mongodump --db database_name --excludeCollection=users
It is also possible to output the dump to an archive file:
mongodump --archive=test.archive --db database_name