Why can't you simply ask the person who made that file to tell you the password?
No one knows how to decode an AES-encrypted file, such as password-protected WinRar files, without knowing the password.
As far as we know, the only way to decode AES-encrypted files is to somehow obtain the right password, and then use that password to decrypt the file.
In the movies, sometimes you see people "cracking" a password.
They try all possibilities for the first letter,
and then when they get the first letter right,
they try all the possibilities for the second letter, etc.
Eventually all the letters fill in, and the then the actor has the whole password.
It doesn't work like the movies with AES. AES passwords are all-or-nothing. If you get all-but-one of the letters right when typing in your password, you get exactly the same "failed" message as if you typed in a completely wrong password.
(It's not possible to distinguish the random-looking gibberish from decoding with an almost-correct key from other random-looking gibberish from decoding with a completely wrong key).
It's not possible to tell if one guess is "closer" to the right password than another guess.
(There are various "timing attacks" that, with some systems, can tell a researcher if one guess is "closer" to the right password than another, but they don't work on AES-encrypted files).
There are various "password unlocker" programs that will try one guess at a time until they stumble on the correct WinRar password.
If the person who made the password-protected WinRar file picked a "strong" password -- for example, a passphrase of 8 words randomly chosen using dice from a short dictionary -- we expect that it will take more than a thousand years to stumble across the correct password.
If that person picked a "very weak" password -- such as a single word in the dictionary, or a single dictionary word followed by a single digit, or a single word repeated 8 times, or a sequence of 5 completely random characters -- we expect that it will take less than a day for that program to run through all those very weak passwords and find the correct one.
According to Wikipedia: WinRAR, the "method of encryption" used by WinRAR is the AES (Advanced Encryption Standard) with a 128-bit key.
I agree with Thomas. It's probably hopeless trying to crack it without knowing anything about the password.
If you do find a way to crack AES, Bruce Schnier, the readers of crypto stackexchange, and every other cryptographer on the planet would be fascinated to find out, especially those cryptographers who work at the SVR, the NSA, and other intelligence agencies.