The application period is from mid-October to early September for Early Decision and to January for most of the other applications. Early Decision means applying to one university earlier, and, in case you are admitted, you are legally obliged to take up the studies (i.e. you must reject the offers from other universities). Universities send their Early Decisions in mid-October and other decisions are sent until the beginning of April. Once you receive the positive decision, you should start applying for a student visa.