Hi @Wilder,
As stated by @simongravelle, this topic has been tackled a lot in the forum and in the archives of the mailing list. You might find answers to your problem by typing lj units in the search bar (we recently gave yet another detailed answer here).
I suggest you read the units command manual page to understand how units are related to one another in lj units and how to convert them back to real units. You only need to choose a mass, a length and an energy scales. The time unit has the dimension of… time so it can be computed both using sqrt(mass*length^2/energy) or sqrt(mass*length*force constant) depending on which one you choose to fix in your parameters.