Because you are a good teacher you know what needs to be taught next in your yearly plan. Therefore you need to keep in mind that the standards that you are planning to teach will need to be covered in your next project. Simple right?
Starting with the standards is all some teachers need to create great projects. Others like to think of a potential project and they see what standards can be taught within the project. This will come down to your personality, most of the time, and will be forced due to the need for certain standards to be taught other times. What is it that you want to teach next? If you truly are new to PBL then there is no harm in looking at other project ideas from places like the Buck Institute or Edutopia.
For example, you want to teach the concept of perpendicular lines and the slope relationship in perpendicular lines. You know that the state standards give you the facts that must be learned, now what? Well, where do you find perpendicular lines? How about where floors and walls meet? How about where there are intersections of city streets? How about irrigation canals? Does it have to be something that is exactly perpendicular? What about light bouncing off of a mirror? Now you can talk about terms like angle of incidence and angle of reflection. They aren't in your math standards? NOW you are seeing the power of PBL - you introduce a project that has students measuring angles of light and they are learning terms that they will need to know in physics.
Let's run with this idea. The students start by measuring the angles of light being reflected off of a mirror. Then they can create the equations of the lines by having the place where the line comes in be the origin of an XY Coordinate System. And, if they draw a line on the mirror what would be the relationships of the light "line" and the line in the mirror? Is there anything unique about the equation of the line that bounces right back to the flashlight? Hey that's a 90 degree angle? They are perpendicular. Is there a relationship there between the two equations?
To make this scenario really work the students will need a "Driving Question." The driving question is quite often in the form of "How does a _____ do _____ by _____ ?" So, for this previous question you might use the question, How does a physicist know that a laser is shining in the right direction by moving the beam along a reflective surface? The driving question must be something that you can come back to over and over again. This helps the students answer the "why are we doing this?" question.
I think you get the idea. You brainstorm ideas that might demonstrate the concept you are wanting to cover. Then you tweak it so that the students have to know particular facts (your standards) to be successful. Suddenly you are being asked to teach them how to calculate slopes of perpendicular lines. They have used inquiry and have come up with a level of knowledge they want and you are providing them the knowledge. It is THEIR knowledge now. They are using it because they want and need it.
Sometimes this brainstorming works best when you are with a friend/co-worker. You might even try asking your "friends" on Facebook or Twitter to help you brainstorm. This might be a good time to start coming to the weekly #PBLCHAT on twitter. It happens every Tuesday night at 9 PM EST and there are lots of PBL folks on there who are more than willing to help you brainstorm. Next time we'll look into your first Entry Event and how you will plan groups for your first project.