The above answer is mostly correct, but I would like to add a little to it to tell you WHY a firm in perfect competition can not influence the market --- WHY it must be a price taker.
The major reason for this is that the product it sells is identical to that of its competitors. Therefore, it can not, for example, raise its price above that of its competitors and claim that its product is superior. If it raises its prices above those of its competitors, it will simply be unable to sell any goods.
Take, for example, eggs which, in America at least, are graded by size and quality. One Grade A egg is essentially identical to another. Therefore, one egg supplier can not possibly claim its eggs are superior to those of other suppliers.
This is why a firm in perfect competition must be a price taker.
One small quibble with the above answer:
In perfect competition, firms may easily enter the market. Because of this, firms in this market structure, by definition, make zero economic profit in the long term. This is why they do not lower prices -- it's not because they have no incentive to do so -- it's because if they do they make negative profit.