Hire an independent valuator. In the case of US companies, a 409A valuation has to occur at every priced financing round, so if you've had one done, that would serve as guide.

But you might not even have the right to sell your shares. Your shares should be subjected to vesting and your voluntary resignation would likely cancel all remaining unvested shares. Even should you have shares to sell, if you have Preferred shareholders, it's likely that there are terms that might make selling your shares to a 3rd party difficult.

Happy to talk in more detail about the specifics of your situation.