get in the closet and make sure you have it light tight, You need it completely dark in there. cover the walls with mylar or flat white paint. No foil.
Decide if you want a soil grow or a hydro grow. that will tell you what kind of chemicals and medium you will need,
Next decide on the light, that will tell you what ventilation you need. I.e. cool tube or some other way to cool the light. that's based on what type of light you get.
Get a thermometer in there and see what the temp is, a light will raise the temp so ventilation comes into play here. We use anything we can to cool our grow areas, that could be an air conditioner or intake and exhaust fans or a combo of the two. then you can start to dial it in by adding a humidifier, fans and odor control.
JP
