How does a perfectly competitive firm decide what price to charge?

A perfectly competitive firm is a price taker, therefore it must charge equilibrium price prevailing in market price, from its buyers. It has no ability to influence market price due to the presence of competitors, it has to accept the market price.

Suppose, in a perfectly competitive market of rice, the equilibrium price is $10$rupee per kg. There is a rice seller who sells its product at $15$rupee per kg. If the seller continues to sell its product at a higher price than the equilibrium price, then it will lose its entire sale to its competitor. This will happen because there are other sellers in the market who sell their rice at equilibrium price which is also known to buyers and buyers will be ready to buy rice at equilibrium price only. Therefore, if a rice seller wants to sell their product, then he or she must accept the equilibrium price.