Get a timer for your lights if you don't have one, also get some sort of reflectors for the walls and roof. Totally light proof the closet no light gets out no light gets in very crucial to have no light leaks.
Get a thermometer and a hydrometer to monitor temp and humidity.
Get a pH test kit and pH up and down.
When you get nutes make sure you follow the charts if you don't have one or are using something like MG then start out at like 1/3 strength and work up to 1/2.
Make sure you have fans for air circulation.
Start with the best quality seeds or clippings for best results, I recommend a clone personally I find its easier to get going on a more mature plant than a seedling as there already rooted and ready to transplant when you get them.
And constantly move your light with your plant don't make them reach for it stay a few inches above tops I use my hand to feel where the heat stops and keep my tops just under that point.
Lastly don't over feed, over water, or under water all can cause stress.
Other than that watch a few grow guides on youtube (great links in this forum Grow videos or free grow videos thread) or read some grow guides online knowledge is key any guide will cover temps, humidity, pH for diff mediums, how to tell when to water ect.
