Get the present as soon as you can control Dink, and watch the story unfold..
You're in a cell now, hit the cell door here and the guard will talk to you, ask for a glass of water and he'll go away. Now push the bookcase to the right to reveal an opening. Enter and go down, right, down and left while avoiding the pillbugs. Check the present if you need some health. There's a save machine and a bottle here that could be handy. Go back out and keep going right, in the end, you'll see a stairway leading up, take it.
Go down and enter the castle. Go right and down the stairs and grab the golden heart that is hidden under the cell, you can kill the goblin, he isn't that hard. Exit the castle and keep going down, you'll get a strength potion and a heart on the way. There's also a house to the right. The woman there will give you some experience if you get rid of the monsters in her basement.
You should go right now and the demo ends there so...
Say that you won't play the full version when the d-mods asks though, for a laugh :)