Is it leaning towards the light? That's normal turn the cup 180 degrees your plant will make itself lean towards the light again the plant is very young and looks fine. Goolge phototropism.
Keep monitoring it over the next few days. It's a slow process for the first few weeks. If the darkening starts to spread, your soil will most probably already contain some nutrients.
Home Depot, Walmart, Lowes, almost any store that sells gardening or home improvement supplies. Avoid Miracle Grow, it contains slow release fertilizer that will make it difficult to control nutrient levels.