For many people starting their work with the Illustrator filling a path with a pattern is really a tough one to figure out. However, there is a quick and easy way to do it:
1. You need a shape (just the stroke, no fill) and a pattern you want to use.
2. Move the shape over the pattern area you'd like to use as the fill. Select both the shape and the pattern.
3. Right click the selection and choose 'Make Clipping Mask'.
4. If you would like to change how the pattern fills the shape, right click on it and choose 'Isolate Selected Clipping Mask'. Then adjust the pattern. After isolating the clipping mask, you can see that the pattern is still a separate object, this is why you can change it whenever you want.
I hope you find this short tutorial helpful Have fun!
P.S. I suggest you used a copy of the shape for clipping mask. This way you can edit the stroke of the original shape.