What should I do if my boss ask me to resign because of exceeding petrol limit?

Yes she can proof it, I only exceeded limit by R1 003, can I be prosecuted for that? am I not suppose to get a warning first?

In every jurisdiction I'm familiar with, converting property belonging to someone else to your own use is larceny and there is no warning first. There are value limits where it changes from a misdemeanor to a felony. Convicting you would require evidence that you did it, I was just going by your statement in the question that, "I've used co. petrol over the weekend, for my personal use." If you end up in court over this, better hope the judge isn't a member of EHelp. :-)

If you live somewhere other than the US, the law could be different, you might want to check to make sure.