What do you mean? It's simple; people got Bitcoin from mining, decided they would sell it if anyone wanted to buy, so they started posting ads, and saying that they would sell their bitcoins for $0.001 each, for example, and others who wanted to buy bought from them, and that's how it started having a market value. If you buy something for $0.001, you will try to sell it for $0.002 so that you can have some profit, and that's what people do.

As time went by, more people came to know about Bitcoin, knew it's a useful digital currency, and as people were already buying and selling bitcoins through p2p deals, it had already gained a market value, and it was increasing slowly because the demand was increasing, that continued, and then exchanges were created, and bitcoins were traded on the exchanges, and developments kept going, demand kept increasing, and the rest is history.