You can start off by taking a short term course on aromatherapy, which you can find at different colleges in most of the cities. Diploma courses offered by City & Guilds, VTCT, etc lead to professional qualifications. You must be at least 18 years or more to practice aromatherapy on a professional basis. You will also find it useful to have a registration as an aromatherapy practitioner. National Occupational Standards for Aromatherapy compliant courses qualify you for the registration. Aromatherapy
Council offers you more guidance and advice.

While practicing, you can upgrade your qualifications and professional development. You can also go for continuing professional development (CPD), which is usually available in the form of short term workshops and courses.