I have some background as an Agricultural student undergrad. Now I study TSI.
I don't think getting "full expertise" is really what you want. Since there are a lot of subjects that will be irrelevant to your intended use in the Agricultural field. Trying to get a full understanding will only disuade you and waste your time.
I would advice you do a research on what are the available tools to solve the problem you are dealing with.
If you have already have decided that R and Python would suit you well. Start with a fundamentals/beginners/first-look course.
The Full Expertise approach would not be fruitful for you since there are tons of material and applications for programming languages.
MIT Opencourseware http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00sc-introduction-to-computer-science-and-programming-spring-2011/