Some paths consider the mental plane to be of 'higher vibration' or less density of energy than that of the astral. It is often found in models of existence involving seven or nine planes, though the most common area I have found the mental plane are in New Age faiths.
I would suggest researching what path you found that mentioned the mental plane and view the differences between it and other planes (as the descriptions can differ from path to path and even person to person). Once you have an understanding of the difference, it will be far easier to reach the mental plane. Practicing meditation will also help tremendously and there are plenty of guides on how to meditate, what it is, and why it is beneficial.