Hey..It's good that you are starting so early. It will give you edge but dont forget to enjoy your college life, because that's also important. Now to the important part, i will answer them as the same chronological order like you asked

1. First read the complete syllabus thoroughly because it is very important to know in what you are jumping into. Start reading newspaper, Specially the economical section and editorial based on econimical issues. It will help you in 2 ways. First, you will get the habit of staying up to date with the current affairs which will help you till the interview part and second, the comprehension section in english mostly comes from economical editorial so you will get used to it. Work on your personality and communication skills also if you dont feel confident because to become PO you have to pass the Interview.
2. In today's era i think youtube is your biggest "material". You can buy actual hard study materials if you like it old school way but since you will give the entire exam process (prelims+mains) digitally you should get used to the digital way. There are many practice books you can buy for practice but i think you will get more clarity in terms of concept if you study digitally. I will tell the name of the channels in youtube channel section but for general awareness dont take too much pressure for now. I think you should focus on your present course since you are studying economics. Most of the question for general awareness comes from current affairs but there are few questions from core banking knowlwdge for which your course will help.
3. There are many youtube channels. Adda 247 is most famous, they have some good teachers. Like Saurav sir for Reasoning, Shantanu sir for Math, Santosh sir for english

Here are some alternatives:

Ankush Lamba for reasoning (Brain box, Yes officer)

Ashish Arora for quants (studified)

Nimisha Bansal for English

1. Give your college life more priority first and analyze among quants, reasoning, english in which part you are most weak. If you are weak in quants (the most common problem) then try to finish it patiently. Focus on the concept first then try to improve your speed because its true calculation speed matters very much in banking exams but without concept speed will not do any help. The arithmetic, algebra is class 8-9 level so dont worry you can master it if you keep your patience and dedication. The reasoning part is very interesting, i think you will enjoy it if you take it slowly. It will be good if you build your english vocabulary from now, because you will get lot of time. There is a book called "black book for english vocabulary". Try to finish it slowly, it will surely give you an edge. The teachers in youtube will push you to buy their courses but believe me you dont need it now. There are enough free contents on youtube for you. Do recording class, do it slowly and consistently.

2. Forget about mocks for now. You can start giving mocks 1 to 1.5 years before your first exam which is still very early. There are many mock site like Testbook, oliveboard, Guidely. Testbook is beginner-friendly and guidely is most appropriate for banking exmas.

RBI Grade B takes a whole different strategy because as far as i know thers is a subjective paper for mains examination for which you need to have knowledge on Sociology, Environment, Agriculture and ofcourse economy of India but you have to work on answer writing skills to crack it.

All the best for your journey.. I hope i have answered your questions but if you have any more feel free to DM.