I believe you would like to know how much roughly it worth before starting telling cosec you want to sell.

Get all yr audited account and look for qualify accountant, get their advice roughly what the company worth or how much yr father put in the $$ in this company.

From there at least you got a figure or amount to negotiate. Tell yr cosec to inform other shareholders you willing to sell off yr portion and request for offer.

Normally if there is fixed asset like building/machine/car or got long term contact, then it may worth some $$

If there is little or no fixed asset, with yr 10% Shareholding, you can't do much. They might dilute yr Shareholding further or just abandoned this company and setup a new company and transfer all business.

You need to play yr card right, seek yr own proper accountant and cosec for advice if you think cosec is on their side.

If you just want this chapter to close, then listen to their offer and if ok, just sell it off in good term with them