Hi J.,
Start with the suggestions below to get ideas about the sun's features. You might also consider a trip to a planetarium, too (such as the one at Griffith Park), or the California Science Center (museum).
You don't mention your daughter's age, so her ability to understand the materials you help her gather must be taken into consideration. Remember it is HER project, not yours. Start with asking her to pose her own questions about the sun. If she can't come up with any, prod her a little with simple questions of your own, such as "How big is the sun? What is the sun made of? What makes it shine?" etc. No question is a stupid question. Every question will lead to an answer and greater knowledge -- that is the basis of science.
Things she will likely learn about are what gases compose the sun (mostly helium and hydrogen), the sun's gravity, heat, what sunspots are and solar flares. She can learn about how these things effect life on earth and our technology.
If she's in middle school, she might want to learn about solar wind, radiation emitted by the sun, why certain types of the sun's radiation are harmful to life on earth (not just humans!), what types are essential, how we can protect ourselves, etc. If she has an eye to the speculative, she might want to learn about other suns in the universe, the possibility that Jupiter might be reclassified as a sun, and if other solar systems might have planets that support life in some way.
As for the project itself, encourage her to be creative. The sun isn't flat, and, if the teacher allows it, her model shouldn't be, either. It's easy to create a sun from a large styrofoam ball, for example, and stick different elements onto it to illustrate the features. A trip to a craft store should generate lots of ideas of things to use, but just looking around the house for odds and ends is likely to allow her to create something just as inventive. Labels can be made from tooth picks and construction paper. Again, remember that this is HER project. You're there to help, not direct, and definitely not to create it. If it ends up looking like something an 8 year old did (and your daughter is about this age), well, that's totally appropriate! No need for you to beautify it. If your daughter creates something that is sloppy for her age and ability, you can suggest that she improve on it, but it is up to her to face the consequences at school.
Have fun with this -- it can be a great project!