There are many excellent zentangle designs online, such as those found at Zentangle.com and on TanglePatterns.com. Pinterest and YouTube are also good sources for zentangle designs.
Many zentangle design websites explain how to create specific designs. They provide tips for creating a new design and offer many examples of already completed zentangles. YouTube features a variety of how-to videos dedicated to zentangle design and drawing. Most zentangle websites rank the designs by difficulty, ensuring that beginning artists do not attempt something beyond their reach and helping more experienced artists to find patterns that are satisfyingly complex. A common feature among these sites is an emphasis on organization, planning and taking one's time with the work. Completing a zentangle should be a relaxing and creative exercise, but it can become frustrating if the tangle is not planned well or if all needed supplies (such as paper, pens, rulers and adequate lighting) have not been considered in advance.
Once you have built up a collection of your own zentangles, post them online, either via your own website or through a site like Pinterest, to share your designs with others, gather feedback from other artists and build your reputation in the zentangle community.