Approach doodling with a relaxed attitude. Doodling is not a high-intensity art form. Many people find that they create their best doodles when they are relaxed and at ease, allowing their creativity to flow.
Practice your own freestyle doodling by drawing circles, squares, triangles, rectangles, waves, swirls and curlicues. Continue drawing these shapes in a variety of sizes. Connect them, create patterns, stack them, line the edge of your page with them and work your way to the center of the paper, changing patterns as you go.
Copy some of the doodles you find online or in books. While you want to progress to creating your own artwork, copying the creations of others will allow you to train your hand to create the shapes and lines that make up doodles.
Buy doodle art books like "Do You Doodle?" by Nikalas Catlow, "The Girls' Doodle Book: Amazing Pictures to Complete and Create" by Andrew Pinder and "Fabulous Doodles: Over 100 Pictures to Complete and Create" by Nellie Ryan. These books offer a jumping off point for fledgling or accomplished doodlers by offering a partial hand-drawn picture with plenty of space for you to complete it with your own doodles.
View some of the many displays of doodle art on the Internet by visiting websites such as Joyfuldia's Doodle Art, Doodle Art Alley and Doodler's Anonymous. The examples on each may fill you with ideas for your own doodles and give you something to aspire to as you progress.