I think your best bet is to use Illustrator. It works much better for patterns. You can Google and find a tutorial easy enough. But, if you have to use PS, you can make a square shape, add stroke and drop the opacity to the inner part so you only see the stroke. Duplicate your layer, scale it down, repeat. I would just duplicate the first layer each time so you can shrink down equal percentages – like the first would be 95%, second would be 90% and so on. Using stroke will help to keep your lines a consistent size. There may be easier ways, but this is what I have offhand.
Good Luck!
TC