More light and ventilation is what you need. First, light.
Marijuana requires 3000 lumens per square foot minimum to grow, optimum levels are around 7-10,000 per sq ft. For reference, the sun provides around 10,000 lumens per sq ft on a clear day. The lights you have in that space will give you 2000 lumens per sq ft, and probably aren't all that efficient, what wattage are they?
If you're on a budget then get some standard light bulb fittings and buy half a dozen 42w, cool white (6500k) compact fluorescents. If you want to get better yields in the long run, get either a metal halide for veg and a high pressure sodium for flowering, or just a HPS, prefferably with an enhanced spectrum bulb. You can even get ballasts that will run either HPS or MH, most digital ballasts have this capability.
For more info on lighting, check here
https://www.rollitup.org/view.php?pg=faq&cmd=article&id=259 and here
https://www.rollitup.org/view.php?pg=faq&cmd=article&id=59.
Ventilation, then. Plants need to breath, just like us, only they take in co2 and give out oxygen. If plants are left in an unventilated room, they will deplete the co2 in the air until they can no longer respire, and stop growth.
So, we need to introduce fresh air into the room. We do this by getting rid of the stale air with an exhaust fan, which should be the centrifugal, vortex or squirrel cage type. Avoid axials, sometimes referred to as inline (although a centrifugal fan can be inline as well) as they will struggle to move air through any ducting or filters.
With HID (HPS or MH) lighting you should be exchanging the air in the room at least every minute, if you stick with flouros then every five minutes may be sufficient.
Work out the volume of your room (LxDxH) in cubic feet and match that to the cfm (cubic feet per minute) rating of the fan. Always get a bigger fan than you need, as ducting/filters will reduce the actual cfm to somewhere below the rating, and fan speed controllers are cheaper than having to buy a bigger fan anyway.
You may well want to fit a carbon filter to the exhaust to get rid of the smell (they will stink once they start flowering). Make sure to match the size of filter to the fan you are using (the filters usually have a cfm rating as well).
Make sure the exhaust is at or near the top of the growspace, and leads outside, or at least somewhere away from the room. The intake hole should be slightly larger than the exhaust, and on the opposite side of the growroom, near floor level.
It's not a bad idea to put an oscillating fan in there as well, the plants like being swayed back and forth, it strengthens their stems and helps maintain an even temperature distribution.
You'll also need some veg nutrients, with a higher ration of N to P and K. Big bloom sounds like an additive to me, not a complete nutrient formula, so you might need some of that as well.
Obviously you'll need something to grow in, either dirt or hydro. Have a look here
https://www.rollitup.org/view.php?pg=faq&cmd=category&id=101 and figure out what you fancy. I'd recommend soil if it's your first time round, it's more forgiving.
You'll also want something to grow, have you got a source of seeds/cuttings?
A thermometer wouldn't go amiss either.
Shit that's a big post. Sorry but I wanted to try and cover everything I could think of. Growing weed is easy if you get the basics right, it can be very hard if you don't have some vital pieces of info. Do some reading, then read some more. When you get sick of reading, do it anyway. I've been researching as much as I can about this for over a year now, and I learn new things every day.
Lay the groundwork, and in a few months time you'll be picking huge nugs from overloaded branches.
All the best.
