Hello blacan0! Welcome to the forums!
One of the prerequisites for the Bachelor of Agriculture @ UniMelb is a math subject - have you studied maths at all in VCE? There are other ways to satisfy the maths prerequisite for this course. From
this page, I have taken that if you have completed Maths Methods 1/2, you can do the Calculus and Probability Online module (which you can see if you're ready for
here or complete the subject MAST10012 during the summer holidays, although keep in mind that these do cost money. There is also the UniLearn course which you can learn more about
here. If you click on the course overview tab, there is a test you can take to see if you have the prerequisite knowledge for the UniLearn course.
It is easier to gain entry to the Bachelor of Agriculture with a Diploma of General Studies. You just need to pass all subjects in the Diploma to gain guaranteed entry into the Bachelor degree. I am not too sure about entry with the Diploma of Agriculture. You can contact the uni about this
here.
Something else to also consider is that completing the Diploma of Agriculture can give you credit for La Trobe's Bachelor of Agriculture and Technology, but this course does not require maths so you can gain admission straight away provided you satisfy its prerequisites (ATAR and English). I would personally study the course at La Trobe and then after a year, if I thought Melbourne's course would suit me better, I would try and contact the uni for details on if/how I could transfer.
I do not study this degree or at Melbourne, so perhaps having a read through the Bachelor of Agriculture page might tell you about the practical to theory ratio and how past students found the degree. You can have a look at things such as the majors and what you will be studying on each course's webpage (for example, La Trobe and Melbourne have different majors in their agriculture degrees which may or may not suit your particular interests in the field). Try not to worry about the difficulty of the course just yet - if you enjoy it, you are more likely to succeed in it.
Hope this helps!

Edit: fixed formatting of links