It's pretty straight forward to set up your units and grid.

open your Preferences window by clicking the button I have highlighted in green, it's found in the bottom right of the interface or through Window > Settings/Preferences > Preferences.

Click the settings menu on the left and you are presented with the working units, linear and angular, set them as necesarry and hit the Save button.

Now open the display menu and open the grid prefs (the little box to the right of the writing) and set the length and width as necesarry (I keep mine around 20) and the grid lines every 1 unit.

you can subdivide the grid, say if you have chosen centimetres as the working unit you may wish to subdivide the grid into mm by setting the value to 10. once you're happy with the settings, hit Apply and Close.

You can now make your model
to scale using the grid as size reference.

Hope that helps,

Mat.

edit: it would appear that Mikee posted as I was posting.