Portals are built by you, you learn how to do it as you get certain items in the game.

Your stamina is gone because you're not being careful with it. Make sure you keep your food up, use the best food you can if you're having trouble with stamina. Very early game you'll want to load up on cooked meat + cooked neck meat + Mushrooms/Blueberries/Yellow Mushroom. And stay rested.

Iron is in the swamp.

You build with stone once you have iron.