Whatever meets these four criteria is a great first car because you'll probably trash it.
1) It's cheap: The cheaper the better on your first car. Don't lock yourself into payments and go for the high end of what you can afford. You still need money to go play around on and dick off with your friends.
2) It gets decent gas milage: Buying gas sucks and expecially when your young and workin only part time.
3) Insurance is cheap on it: you have the right idea looking at 4 doors. The insurance is generally cheaper. Cheaper cuts out more payments and adds more money to dick off on.
4) It is decently reliable: something that will last you a few years and don't buy a car just because it looks pritty. Like I said you'll probably trash it. I'm not trying to affend you but I was young, my friends were young, and we all trashed them.
Something not super fast is also a good idea because you'll be less likely to crash and it will generally meet this criteria. You won't be less harsh on a slower car but the previous owner probably was unless it was another youngen and I will probably have less abuse on it.
Don't be mad at your sister for having criteria for you to pass to co-sign a loan for you. It is a big responsibility for her and if you fuck up it falls on her shoulders. You should be happy to do what she wants and kiss her ass for this trade off. My best friend got fucked over by co-signing for his brother and him not paying and shit.
In short, buy a cheap reliable car regardless of if its exactly what you want. You will have plenty of time later in life to buy what you want.