How do i make Models rotate/turn with a script?

You are correct on that note. To Rotate all children of a model, I'd recommend using the GetChildren method.

a = game.Workspace.Model
num = 0
repeat
for i,v in pairs(a) do
    v.Rotation = v.Rotation + .1
end
num = num+1
wait()
until num == 10

The above script will move all the parts in a circular motion until I call it to stop.

A way I commonly use involves Model:SetIdentityOrientation() and Model:ResetOrientationToIdentity().

First, set the model's PrimaryPart to something inside the model (a Part will work) In this case, I will use part P.

Now we call Model:SetIdentityOrientation().

Now, I set P's Rotation value to a rotation I choose. Note that the model will rotate the other way.

Finally, we call Model:ResetOrientationToIdentity() to rotate the model.

From here you can reset the PrimaryPart value if you wish, if so you should call the Destroy() method on part P.