Most importantly, you need experience managing a farm. You can start of as an assistant manager or manager of a specific area like livestock or arable. Farm managers usually have some levels of qualifications in agriculture. BTEC National Diploma in Agriculture and several other degree programs in agriculture or related subjects can get you qualified. You can also further your qualifications after you start working as a farm manager. You can opt for NVQ in livestock, agricultural management, etc. You can also opt for degree or postgraduate qualifications in agriculture or related subjects. You can find part time and distance learning opportunities.