What does the "Warp" option in the menu do?
It makes the game harder.
Actually, no, it doesn't do anything.
For the rest, as Astro said, it's pretty straightforward. Click where you want Sam to go and he'll walk there. If the cursor gets a green outline when it moves over an item or person, that means you can interact with the item or person by clicking. The gun can be shot at anything you feel like.
Access your inventory by clicking the cardboard box in the lower left corner, and then you can "pick up" an item by clicking on it. Your cursor will change from an arrow to that item, and you can use the item on another item or person by clicking while your cursor is over that item or person. Right-click to turn your cursor into an arrow again.
Access the menu by clicking the gear icon in the upper left corner, or by pressing Esc on your keyboard. In there you can save/load the game, change game options, and quit.
Oh, and if you want to skip through dialogue more quickly, use the spacebar or the right mouse button.
Hmm... did I forget anything?
As far as what you're supposed to do, just let the story unfold. The point of the game is to progress the story and find a way around any obstacles that arise (by using inventory items, talking to people, etc.)