General advice below. Everyone will vary.
This will be an ongoing thing. Ideally you want to max out your 401k/IRA every year. May not be feasible at 21, but start at something. Then every time you get a raise, increase your contribution amount just a little bit. Since you got a raise at the same time, your income will go up and you won't even notice the extra amount going to your retirement fund. No need to wait for a certain amount to be in a retirement fund.
rainy day/insurance/other payments and God forbid divorce
The general rule is 3-6 months of expenses in a readily available account. Look at your expenses in a typical month - rent, utilities, insurance, food, normal life expenses - multiply it by 3-6. Also since you mentioned credit - pay off your cards every month and never be late on any payments. Do that consistently for years.
I don't know that I've ever heard anybody saving for a divorce. Probably shouldn't get married if you're planning to be divorced
Then when it comes specifically to a car like you're considering, remember that operating expenses are going to be more than a normal car. Hit a pot hole and need to replace a tire? $500. Got a used one that is out of warranty? Lots of expensive things can break. Insurance is going to be a lot more. And it all gets worse if you modify. You never want to be in a position where you're struggling financially to keep your car on the road. It isn't worth it. Spend money on experiences, not objects.
There are plenty of still fun but much cheaper cars out there. Start with one of those while building up your savings. If you take out a loan on that car, it will start building up your credit. Then you get to enjoy your car and it becomes part of the down payment for the next car.
