Please credit Andraqua of TrueAchievements.com for the following solution:
Head into the Tutorial Level and bypass the starting area by hitting 'B' when prompted. Make your way up the stairs and head into the brewing area on the right. Open the chest and take the bucket of water (this will be explained later). Next, head into the Enchanting area (again on the right just after the brewing area) and open the chest on the right and collect a shovel, a pickaxe and an axe. Leave the Enchanting Area and turn right. You'll come to an area that shows you how to build Iron Golems. Use the pickaxe to break one of the blocks of iron and add it to your inventory. Next turn around and head down to the area with the farming. Here you'll want to use your shovel to break all the gravel at the bottom of the stairs until you find a piece of flint. Take the flint and head back upstairs. Finally for this section, use your axe to destroy one of the buildings. You'll need 4 wood planks to create a crafting table. Once created, convert the Iron Block you collected earlier to Iron Ingots. Now you can create the Flint & Steel (the item that activates the Nether Portals).
Now to head to the Nether Portal. Go back to the area where you retrieved the Iron Block and head up the stairs, bypassing two Ender Chests. Follow this circuit round until you see a narrow staircase leading down ahead of you. Head down there and the Nether Portal with be in a building on your left. This portal is already activated but this is where the bucket of water comes in handy. Empty the water on one of the obsidian blocks at the bottom of the portal and this will deactivate it. Collect the water again. Now equip your Flint & Steel and re-activate the Nether Portal.