Assuming you are not having a separate grow and bud chamber,

for fluoros: you'll need at least one fan sucking in air from under your door, and one blowing it out the top. You'll also want an oscillating fan to move air around inside.

for HID: you will need a fan to move air around inside, plus a dedicated ventilation system just for your light (unless maybe you are using a 150 watter or something), preferably a cool tube or enclosed hood, plus sucking in low and pushing out high (heat rises).

If you use a cool tube or enclosed hood, you could have a 600 watt HID, or maybe even 1000. With HID you will need to either do HVAC calculations, or just do what seems right and make a few changes along the way, like I did. Always over estimate what you think you'll need when it comes to ventilation.

Whatever you choose, odor will be an issue. You'll probably want a carbon scrubber.