It is nearly impossible to crack password for a rar file, you can only try to remember it. I had once forgotten the password to a personal rar file that I myself had locked; the password was strong, and I remembered all but few letters. The stuff was really important to me and I tried everything in the world but could not break the password despite of the fact that I knew most of it. Though I recalled the password at last.
However, if the password is weak, you can attempt a brtue-force attack using the standard tools. Any tool that claims to offer a 100% success is a scam, since the whole point of encryption (AES for ex.) is that with a good password, even the best supercomputers of our time won't do the job.