Ask the first graders what they think of when they think of or see the word "art." Get an idea of what the class' view is of art. For instance, they may think of garden sculptures or the pictures they draw at coloring time.
Gauge the classes interests. If you introduce watercolor painting, for example, some students may really like it, while others would much prefer using oil pencils or drawing sketches with pencils. Introduce a few art ideas to the first graders---painting, drawing, pottery, or collage, for instance---and show examples of each. Let the class as a whole pick which art subject they would like to start with first.
Show a few of your paintings, drawings or sculptures to the class. If you are an artist yourself, showing the class your work makes art seem more accessible and achievable than looking at textbook examples of classic paintings (though you can show these off with enthusiasm later). Describe your process of creating the art without instructing students "how" to create the same thing.
Go to an art museum. Let students see paintings and other artwork in the real world. If the museum offers tours for K-12 students, call ahead to reserve a time for your class. Sometimes these tours include drawing or painting exercises. If the museum has an outdoor sculpture garden, be sure to take the class out to walk around and view, and sometimes touch, the art so they can release some energy.
Invite students to mimic a painting or other work of art they liked at the museum. Provide students with a bunch of supplies, from paints to chalk to crayons to yarn. Ask the community for help: start a drive to help bring in donated supplies to your school.
Practice. When you introduce a technique, such as contour drawings, let students have hands-on experience before starting a full project. If you are teaching the first graders about "blind contour drawings"---drawing a portrait or object by following the line with the eye but not looking at the drawing surface--- encourage students to ask questions about the process.
Ask questions to the students about how a certain technique was achieved. For example, you can ask "How do you think the artist created this pine tree?" The students answers may not be correct, but it will get them thinking about certain brush strokes and shading effect pieces of the painting.