If you are thinking about a career at school or are about to leave school with A-levels, university might appear to be the choice as this is the traditional path for the professional adult and will take several years. Normally 3-4 years for a first degree and more if you wish to study for an MA. If you are past this time you may consider part-time educational courses that will give you an equivalent vocational qualification.
Alternatively, there are some very good private design academies, colleges, schools that have an excellent reputation for shortening the process, with greater focus on the practice of design as opposed to design education. The top pages of Google, Bing and Yahoo will give you a choice of these organisations, many of which are in London, for example, Inchbald, Interior Design School, JJAADA Academy, KLC. There are now many organisations that offer online programmes.