Use the Colors & Patterns section in the properties panel palette to change the fills and strokes of your objects. To apply a pattern to an object via the Swatches panel / palette, select the object, then ctrl click (right click) the pattern you want to apply, then simply apply to the fill or the stroke of the object. You can also apply the pattern from the properties panel / palette by selecting the object, choosing the fill or stroke picker, then choosing Pattern and navigating to the pattern you wish to apply. Any patterns you have created will also appear here underneath the default ones. Once you select a pattern it will be applied to the object immediately in the document.