When space limitations decrease available growing area, you need simple strategies to reduce frustrations and increase harvests in even the tightest closet setup.
Many of the smaller grow areas I see suffer from the same problem: The growers lose focus and make compromises regarding one or more of the major factors of successful cultivation. They may provide plenty of light yet completely ignore air circulation; thus their plants suffer in hot, spent air. Some move plenty of air in and out but skimp on proper horticultural lighting and therefore grow long, lanky twigs. Others constantly overwater and wonder why their leaves droop.
Growing in smaller spaces doesn't have to be an exercise in futility. In size-compromised environments, it's especially important to maintain appropriate levels of light, water, nutrients, pH and movement of air. These major factors will determine whether you'll succeed or have to continue learning from your mistakes.
Before you begin growing, the closet you choose must be thoroughly cleaned out. Remove all items that aren't related to your purposes, and paint the walls with a fresh coat of white paint for reflectivity. Cover up any cracks or holes, and waterproof the floor and at least the bottom 6 inches of the walls with a tarp or plastic sheeting to prevent potential leaks.
Always keep the closet area free of dirt, dead leaves and other debris to limit pests and potential molds or other maladies. Many of the grow spaces I've seen, both large and small, suffer from a lack of proper hygiene.