Ideally, try and get a job in the industry for a summer, or get a bit of experience before you go into it. You have to be happy with working outside, and doing physical work. If you are not prepared to work hard or are looking for a soft job, don't go into Landscaping. Design is very sexy at the moment, everyone wants to be a designer, a Landscape Designer. It's different on the ground, you have to be out there on sites in all weather and you have to make sure projects are managed well and you're able to muck in with everyone else.
Biology is most important for anyone going into Horticulture or Landscaping as it covers propagation and helps with the identification of plant names, species and families through the universal use of Latin. Chemistry is also helpful as the use of various chemicals is a constant in horticulture. The chemical content and dangers of fertilizers, herbicides and insecticides in use in Amenity Horticulture needs to be understood anyone going into this business. Geography would be a relevant subject as well.
Also, the simple things like having a full, clean driving licence, which can make you a lot more employable if you are trying for a job with a Landscape Conractor. This indicates that you are more mobile and can also drive a company van if needed. Be sure you're happy with the outdoor life. Having taken a Horticulture course will give you an advantage. However, it's possible to take a job first and study later, e.g. in IT Blanchardstown it is possible to study at night. I think you cannot beat doing the Diploma Course in the National Botanic Gardens because it is a good practical course which also covers all the theory and is invaluable for gaining plant knowledge.
Enterprising people like situations that involve using resources for personal or corporate economic gain. Such people may have an opportunistic frame of mind, and like commerce, trade and making deals. Some are drawn to sales and marketing occupations. Many will eventually end up owning their own business, or managing a section in larger organisations. They tend to be very goal-oriented, and work best when focused on a target. Some have an entrepreneurial inclination.