How is the winner/s determined
It is not that kind of game. You don't have winners and losers. The best kinds of games are the kind where the players collaborate, not compete. You survive or you die. You prosper or you do not.
Let me try to explain it better. It is a fantasy game. That is, you as a group participate in a fantasy. The DM creates the world/ setting. He sets the stage and fulfills the role(s) of the universe. The rest of the players interact with it.
The guys who invented d&d based it on Lord of the Rings
, which is a good example of how a game scenario may play out. You, the players, have adventures like the hobbits did.
In some cases the DM says "this is your goal" in other cases, goals are entirely player driven ("I want to build a castle/ steal a gem/ marry the princess/ crush my enemies"). So, you sort of create a story as you go. It is a lot like improv, only there are rules for resolving conflict. If you wish to fight someone, there is a system to determine who wins the combat. If you wish to climb a wall or avoid a trap, there are game mechanics for that, all of which involve rolling various dice.
A game that lasts more than one session is called a campaign. In a campaign, you will have more than one adventure. Later adventures are direct results of the prior ones.
and how does a game end. how long does a typical. game go for??
It depends. I ran a campaign that lasted for 10 years. A given game session would last 4-6 hours. We would stop when we got tired or had a good place to leave off.