Too many unknowns to really give you a good answer. What size of studio? How powerful are the lights? Do you want to go highkey or lowkey? Is the model light or dark skinned? Do you want hard or soft light?
So to get a better idea about what you are trying to do, can you post a link or photo that illustrates what you're after.
In my example of Haley, I wanted soft light, highkey, even lighting so I used a large softbox.
Re: What modifier to use?
Cheers,
rfs