I find it helpful to lay out the game in terms of goals desired to be achieved, and then fit in obstacles to those goals to create the puzzles.
So the end-goal here seems to be "Get the girl".
To get the princess, he needs to:
Gain back some strength.
Find out where the princess is.
Get to the princess.How will he gain strength?
He can get some blood from the teens.
How will he get blood from the teens? (He's a bit too weak to attack them right now)
He trades them something.
What does he trade? What does a goth teen want?
He gives them his spiffy black collared cape in exchange for some blood.How does he find out where the princess is?
He asks around. Some people direct him to go to the museum or library.
The library gives the location, but the door is sealed, and the library makes mention of some artifact needed to open the seal is in the museum. Getting into the museum requires purchase of a ticket.
How does he get money to pay for a ticket?
He does his puppet routine on the long line of people waiting bored to get into the museum. Some of them toss him some coins, which he uses to buy a ticket.
Once inside, how does he get the artifact from the display case guarded by a guard and security camera?
He's a vampire, so his image doesn't show up on cameras, and he distracts the guard with a puppet show (again?), and nabs the artifact, and uses it to get into the sealed castle with the princess.
How does he free the princess from the clutches of the monster in the castle?
PUPPET SHOW AGAIN!
You obviously wouldn't be able to use this one, it is far too short and too many of the same puzzle solutions, but perhaps it can give you some ideas. I sort of morphed it off Rodekill's old puzzle tutorial, you can find a version of here
Still, I'd say the basic structure would be the same. Take plot-points, convert them into the result of obstacles that have been overcome, and then divide those obstacles into further obstacles. So:
How to gain back strength to travel about?
Get blood from goth kids.
How to get blood from goth kids?
How to find where princess is?
Check the local library/museum/professor of history.
How to get to those locations?
How to free princess
Also get info on how to from library/museum/professor.
How to get items required to get to princess?
And so on. At least that is how I do it