My recommendation would be, like already mentioned, pick something you are comfortable with, and pick something that doesn't bring up any associated memories or thoughts as these just act to distract you from your meditative state. When people learn TM they are given a mantra which, whilst it does actually mean something, it's not in their own language and is not likely to have any meaning to that person, so it's just something that is a 'sound' to them, without any identification.
Hugs
Giles
