Hey Julian! Welcome to bigger pockets! You're asking some good questions:
1) It's always better to have more money saved up. So no matter what my answer to this question is, you should do everything in your power to have as much capital as you can. You can invest with no money down, but people are less likely to loan to beginners with no "skin in the game". How much money do you need? Completely depends on your market and your goal and your strategy. With BRRRR, you want to ideally buy the entire house with cash, so that might be 50k-100k in Detroit or Baltimore, but 500k-1M in LA, for instance. With house-hacking, you can get by with only a 3% down payment on the low end, so that could reasonably be only $5000 down payment in some markets.
2) Working with a partner has pros and cons. I would advise working with a partner if the two partners have very complimentary skillsets, otherwise you get the too many cooks in the kitchen problem, which is amplified when there's a lot of money at stake. If you're not sure, I would lean on the side of going solo.
3) Start by finding an agent. Find a good one - they're the one that should guide you through the process. They would show you the house and if you're interested, they would write up the contract for you to sign, etc.
4) There are lots of books and forum posts on how to screen your tenants. Or get a good property manager.
5) A rehab job should take 2-4 months depending on how large the project is.
6) With a BRRRR you should be buying in cash on the front end, so your credit score is irrelevant. On the backend, after you've rehabbed, a good credit score will get you a better rate for your cash out refinance. Aim to get your beef up / fix up your credit score regardless what real estate strategy you want. See Graham Stephen on youtube for some tutorials on how to best do that.