Hi there! Don't be afraid - everyone starts somewhere and you've got to make the leap. I understand it can be daunting to start, but once you do, you'll be set.

My number 1 advice to anyone starting UCAT prep is to
never, ever, dive straight into timed practice. Think about it - you don't know how to approach the different types of questions present in each section accurately and don't have any built-up speed. This is similar to completing a VCE exam at the start of the year when you have no knowledge of the course since you haven't been taught. Obviously, the 'content' you need to know for the UCAT exam is diametric to a VCE exam, since it can easily be self-taught and can be done in a much shorter timeframe required for the VCE.
Medify has short informational sections that address the basics of each question type in each section. Read these to get a basic idea, and attempt a few hundred questions from each section. I also recommend Medic Mind and Kharma Medic - 2 great youtube channels if you are struggling to hone your technique/ways of approaching a question. After you complete each question in untimed conditions, go over it thoroughly. Note down any repeated mistakes you make, and which techniques seem to work for you. For some sections, it may take more or fewer questions to feel familiar enough to start timed practice, and that is okay. You need to keep doing untimed practice until you feel comfortable with all question types that can appear in a section.
Accuracy is always the first step to conquer, never speed. Speed comes second, once you can accurately answer the questions. Obviously, this doesn't mean you need to always get 90%+ on each practice drill. It just means you can do well on the questions and feel that your results are pretty consistent. Once you are in the middle to end of untimed practice, try not to take too long on the questions you complete. For example, if it takes you over 2 minutes to do an AR question, you should go back to the basics and learn more techniques/methods until you can answer the question in a shorter period of time. 'Untimed' doesn't necessarily mean take as long as you want, it just means going slightly overtime is not penalised as it is in timed, giving you the opportunity to figure out the way you can approach the questions that makes most methodological sense to you, and results in accuracy.
Untimed practice shouldn't take you too long, and the majority of the time you spend should ideally be the time you spend doing timed practice. The main difficulty of the exam stems from its time constraints, so it's important you learn how to maintain accuracy in these restricted conditions. You can start your mocks whenever you feel you have adequately prepared to attempt a mock. This is best, since the mock will, in this case, give you a more true indication of where you sit, rather than if you sit it when you aren't as fully prepared as you could have been.
As an example, I try to aim for 1 hour of practice, which usually amounts to 100 questions a day, but this really depends. If I'm focusing on a section like VR, I usually only complete 70 to 80 questions. But if I'm doing SJT (which I often couple with another section, since SJT is a section I can finish quickly with time leftover) I often complete around 150-200 ish questions. If I feel guilty since I haven't practised for a while, I'll try to do many more questions. Some days I'll only do around 50-ish questions. Again, this stuff is different depending on the individual. Try not to put a number on the number of questions you do. There are also school commitments. If I have a SAC the next day, I won't be sitting down and doing 100 UCAT practice questions, because for me that's not feasible for that day. Some people will focus on one subsection a day, some will do more. This again depends on the person. Despite this, always remember to spend sufficient time going over what you got wrong and making sure you improve from your mistakes - this is important!
UCAT prep is really based around you as an individual, and there is no hard and fast rule that guarantees success. Everyone learns differently and at different rates, so take your time to go through and familiarise yourself with everything!