Fiber is found in fresh, unprocessed foods. Eating these foods will help you increase your fiber intake. Fruits and vegetables are good sources of fiber as well as many other nutrients. When you use these for fiber, don't peel them since a lot of the fiber is located there.
Nuts and seeds are also good sources of fiber but contain a lot of calories. If you're trying to lose weight get your fiber elsewhere. Whole grains also contain an abundance of fiber. Make sure that when eating these you look on the ingredients list. Check to see if 100% whole wheat flour is listed first. If it isn't, find one that is since these will contain the most fiber.
There is also wheat germ and wheat bran. These are taken off of the wheat when white flour is made. Most of the fiber in wheat is located in one of these. You can buy them in a bottle and sprinkle them over any food or add them to your recipes.