There are 3 things to remember with pattern:
1. Color
2. Scale
3. Design
Let's talk about color first. Most pieces of fabric will have an overall color. If you have trouble seeing it, do the squint test (or if you are as nearsighted as I am, just take off your glasses) When you look at the fabric and squint, you will see the dominant color in the fabric. The color you see should be in harmony with the colors you know look good on you. Some people look terrific in prints on a white background, others look better with a cream or black background.
Scale is how big the print is. For instance, I am crazy about plaid. I'll wear it anywhere on my body. However, it has to be a very small checkered type design or it overwhelms me. The same thing is true for floral prints, geometric designs, etc. Whether you look better in small or large prints will depend on your facial features, your coloring, and your body structure. You can be drawn to an item of clothing because of the color and style, but if the scale of the pattern if wrong, it may end up not being worn.
When I talk about design, I am talking about the lines in the garment. Are they straight or curved? In plaids they are always straight. Floral prints can have gently curved lines, or be very round, depending on the design. Geometric prints vary (circles, squares, triangles, etc), and abstracts can be anything.
Again, what you will look good in will depend on the lines of your body. Is your face angular or round? Is it both? How about your body? Again, do the squint test if you are having trouble trying to determine you body shape.
Develop your sense of pattern. It is just one more tool that will help you to avoid those costly buying mistakes that hang in the back of your closet.